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OCR for page 3447
Proc. Natl. Acad. Sci. USA
Vol. 96, pp. 3447-3454, March 1999
Colloquium Paper
This paper was presented at the National Academy of Sciences colloquium "Geology, Mineralogy, and Human Welfare, "
held November 8-9, 1998 at the Arnold and Mabel Beckman Center in Irvine, CA.
Manganese oxide minerals: Crystal structures and economic and
environmental significance
JEFFREY E. POST
Department of Mineral Sciences, Smithsonian Institution, Washington, DC 20560-0119
ABSTRACT Manganese oxide minerals have been used
for thousands of years by the ancients for pigments and to
clarify glass, and today as ores of Mn metal, catalysts, and
battery material. More than 30 Mn oxide minerals occur in a
wide variety of geological settings. They are major components
of Mn nodules that pave huge areas of the ocean floor and
bottoms of many fresh-water lakes. Mn oxide minerals are
ubiquitous in soils and sediments and participate in a variety
of chemical reactions that affect groundwater and bulk soil
composition. Their typical occurrence as fine-grained mix-
tures makes it difficult to study their atomic structures and
crystal chemistries. In recent years, however, investigations
using transmission electron microscopy and powder x-ray and
neutron diffraction methods have provided important new
insights into the structures and properties of these materials.
The crystal structures for todorokite and birnessite, two of the
more common Mn oxide minerals in terrestrial deposits and
ocean nodules, were determined by using powder x-ray dif-
fraction data and the Rietveld refinement method. Because of
the large tunnels in todorokite and related structures there is
considerable interest in the use of these materials and syn-
thetic analogues as catalysts and cation exchange agents.
Birnessite-group minerals have layer structures and readily
undergo oxidation reduction and cation-exchange reactions
and play a major role in controlling groundwater chemistry.
Manganese (Mn) is the 10th most abundant element in the
Earth's crust and second only to iron as the most common
heavy metal; on average crustal rocks contain about 0.1% Mn
(1~. Geochemically, Mn behaves like Mg, Fe, Ni, and Co and
tends to partition into minerals that form in the early stages of
magmatic crystallization. Significant quantities of Mn persist,
however, in melts and can be plentiful in late-stage deposits
such as pegmatites (2~. Mn is readily depleted from igneous
and metamorphic rocks by interactions with surface water and
groundwater and is highly mobile, as Mn(II), in acidic aqueous
systems (2~. Near the Earth's surface, Mn is easily oxidized,
giving rise to more than 30 known Mn oxide/hydroxide
minerals. These oxides are the major players in the story of the
mineralogy and geochemistry of Mn in the upper crust and the
major sources of industrial Mn.
Most people's introduction to Mn oxides is the messy, black
innards of a dry-cell battery. But in fact, Mn oxide/hydroxide
(referred to generally as Mn oxide) minerals are found in a
wide variety of geological settings and are nearly ubiquitous in
soils and sediments. They occur as fine-grained aggregates,
veins, marine and fresh-water nodules and concretions, crusts,
dendrites, and coatings on other mineral particles and rock
surfaces (e.g., desert varnish). Because Mn oxides commonly
form at the interface between the lithosphere and hydro-
sphere, atmosphere and/or biosphere, they can provide envi
PNAS is available online at www.pnas.org.
ronmentally relevant insights into certain types of interactions
between these systems and potentially serve as long-term
monitors of changes within a system.
As ores, Mn oxides have been exploited since ancient times.
In particular, pyrolusite (MnO2) was prized as a pigment and
for its ability to remove the green tint imparted by iron to glass
(3~. By the mid-19th century Mn was an essential component
in steel making, as a deoxidizer and desulfurizer and for
making hard-steel alloys. Mn oxides are the predominant ore
minerals in most of today's commercially important Mn de-
posits, commonly formed by weathering of Mn-rich carbonates
or silicates, either by in situ oxidation or by dissolution followed
by migration and reprecipitation (4~. Approximately 80-90%
of the current world production of Mn ore is consumed by the
steel industry; on average, steel contains about 0.6 weight
percent Mn but may be 10~o or more in high-strength steels (5~.
Other uses include production of special Al alloys, Mn chem-
icals, catalysts, water-purifying agents, additives to livestock
feed, plant fertilizers, colorant for bricks, and in batteries.
Natural Mn oxide (primarily nsutite) is used as the cathodic
material in zinc-carbon dry-cell batteries. In recent years,
however, alkaline batteries, that use synthetic, electrolytic Mn
oxide, have increasingly dominated the market (6~.
Ocean Mn Nodules
The most extensive deposition of Mn oxides today occurs in the
oceans as nodules, microconcretions, coatings, and crusts (7~.
Marine Mn nodules were first discovered in 1873 during the
voyage of the HMS Challenger (8~. Since then, Mn nodules
have been found at almost all depths and latitudes in all of the
world's oceans and seas (7~; it has been estimated, for example,
that they cover about 10-30% of the deep Pacific floor (9~.
Ocean Mn nodules typically are brown-black and subspherical-
botroyoidal and consist of concentric layers of primarily Mn
and iron oxide minerals. Other minerals commonly found in
the nodules include: clay minerals, quartz, apatite, biotite, and
feldspars (10~. Most Mn nodules have formed around central
nuclei that may be carbonate mineral fragments, pumice
shards, animal remains, coral fragments, etc. (11~. The nodules
range from 0.5 to 25 cm in diameter, with an ocean-wide
average of about 4 cm (11~. Marine Mn oxide crusts and
nodules concentrate at the sediment-water interface (12) but
locally are distributed to a depth of 3 or 4 m (13~. The nodules
are most abundant in oxygenated environments with low
sedimentation rates and reach their greatest concentration in
deep-water at or below the calcium carbonate compensation
depth (11~. Accumulation rates range from 0.3 to 1,000 mm/yr
in near-shore environments to about 1 cm/million yr in the
deep ocean (14, 15~. The source of the Mn is thought to be
continental runoff and hydrothermal and volcanic activity at
midocean spreading centers (16, 17~.
Abbreviation: TEM, transmission electron microscopy.
3447
OCR for page 3448
3448 Colloquium Paper: Post
Research on the complex mineralogy of the Fe and Mn
oxides in ocean Mn nodules has been hampered by the fact that
the minerals typically occur as thin layers of fine-grained,
poorly crystalline mixtures. Previous studies of the mineralogy
of ocean nodules concluded that the dominant Mn oxide
phases are birnessite (7 A manganate), todorokite (10 A
manganate), and &-MnO2 or vernadite (18~. Both birnessite
and todorokite commonly are found in the same nodule, but
birnessite tends to predominate in nodules from topographic
highs such as seamounts and ridges, and todorokite is more
common in slightly more reducing near-shore and abyssal
environments (19, 20~.
How ocean Mn nodules grow is a subject of intensive
research and some debate. Nodules apparently grow princi-
pally by direct precipitation of Mn from seawater, but the types
of reactions that occur in the water and at the precipitation
surface are poorly known (2, 17~. It also has been suggested
that some Mn and Fe is supplied by upward diffusion through
underlying reducing sediments (2~. One scenario suggests that
Mn oxide phases in ocean nodule form by catalytic oxidation
and adsorption of Mn(II) on suitable substrates, such as
mineral and rock fragments and fine-grained MnO2 and
Fe(OH)3. Once initiated nodule formation is self-perpetuating
because Fe and Mn are autocatalytically precipitated on the
surface (24. Indeed Mn oxides, and Mn nodules themselves,
have been recommended as oxidation catalysts for automobile
exhaust systems (21) and for the reduction of nitric acid
pollutants (22~. It also has been proposed that in some
environments bacteria might be the dominant catalysts for Mn
oxide precipitation (7~. For example, Mn-oxidizing and Mn-
reducing bacteria isolated from deep-sea nodules have been
shown to increase experimental deposition of Mn onto pul-
verized nodules (23~. More recent studies (summarized in ref.
24) indicate that microorganisms can accelerate the rate of
Mn(II) oxidation by up to five orders of magnitude over abiotic
oxidation, and thus are likely responsible for much natural
Mn(II) oxidation.
Ocean nodules are of potential commercial interest because
in addition to Mn they also contain significant amounts
(several tenths to more than one weight percent) of Cu. Ni, Co,
and other strategic metals (e.g., ref. 254. Laboratory experi-
ments have shown that the sorption capacity of freshly pre-
cipitated Mn oxides is extremely high for a variety of metal
cations (26-28~. Thus in seawater, adsorption by Mn oxide
deposits may be the most important mechanism for controlling
the concentration of heavy metals (27~. The relatively slow
accretion rates for deep-sea nodules provide ample opportu-
nity for adsorption of heavy metals and is consistent with
observations that more rapidly growing nodules tend to have
lower trace metal concentrations (17~. It is unclear how the
heavy metals are bound in the nodule Mn oxide minerals, or
even in which phases they are concentrated. Experiments have
demonstrated that adsorption of heavy metals by hydrous Mn
oxides is accompanied by release of protons (H+), suggesting
that the cations are bound into the Mn oxides' atomic struc-
tures (27~. Additional insights into the nature of the heavy
metals likely await a more detailed understanding of the
atomic structures and crystal chemistries of Mn oxide minerals
in ocean nodules.
Mn Oxide Minerals and the Environment
The unusually high adsorption capacities and scavenging ca-
pabilities of Mn oxide/hydroxide minerals provides one of the
primary controls of heavy metals and other trace elements in
soils and aquatic sediments (28, 29~. Understanding such
controls is important for maintaining and improving fertility of
soil, mitigating health affects in humans and animals, and for
treatment of water for consumption and industrial use. Be-
cause Mn oxide minerals commonly occur as coatings and
Proc. Natl. Acad. Sci. USA 96 (1999'
fine-grained aggregates with large surface areas, they exert
chemical influences far out of proportion to their concentra-
tions (28~. The presence of only tiny amounts (e.g., a fraction
of a weight percent of soil or sediment) of Mn oxide minerals
might be adequate to control distribution of heavy metals
between earth materials and associated aqueous systems (28~.
Additionally, Mn oxides can act as important absorbents of
phosphate in natural waters and surface sediments (30~. Two
useful applications of the scavenging ability of Mn oxide
minerals are as geochemical exploration tools (25, 31, 32) and
purification agents for drinking water (33~. Recent studies also
indicate that Mn oxide minerals in soils and stream sediments
and as coatings on stream pebbles and boulders might serve as
natural traps for heavy metals in contaminated waters from
mines and other industrial operations (32, 34, 35~. Similarly,
Mn oxide absorbers effectively recover Ra, Pb, and Po from
seawater (36), and it has been shown that the geochemical
distribution of several naturally occurring radionuclides
(234Th, 228Th, 228Ra, and 226Ra) is controlled by Fe and Mn
oxides (37, 38~.
Hydrous oxides of Mn occur in most soils as discrete
particles and as coatings on other mineral grains. Mn is highly
mobile in acid, organic soils of the temperate and subarctic
zones, but in the more alkaline tropical soils Mn might
concentrate with residual laterites (24. It has been noted that
frequently observed influences of pH, organic matter, lime,
and phosphate on heavy metal availability in soils are under-
stood principally in terms of their influence on the chemistries
of hydrous oxides of Mn and Fe (284. The major Mn minerals
reported in soils are lithiophorite, hollandite, and birnessite
(39, 404; it is more typically the case, however, that because the
Mn oxides are fine-grained and poorly crystalline (commonly
referred to as amorphous) that no attempt is made to assign
mineral designations.
Mn oxides in soils and sediments readily participate in a wide
variety of oxidation-reduction and cation-exchange reactions.
They exhibit large surface areas and can be very chemically
active. Birnessite directly oxidizes Se(IV) to Se(VI) via a
surface mechanism (41), Cr(III) to Cr(VI) (42), and As(III) to
As(V) (43~. Certain Mn oxide minerals easily oxidize arsenate
(III), the more toxic form of inorganic As, to arsenate(V),
which can more effectively be removed from drinking water by
existing water treatment procedures (44~. Mn oxide minerals
such as birnessite and todorokite readily undergo cation-
exchange reactions (45), and studies have shown that the cation
exchange capacity of Mn dioxide at pH 8.3 (about that of
seawater) exceeds that of montmorillanite (46~. Clearly, these
kinds of reactions profoundly affect the chemistries of soils and
associated aqueous solutions.
Mn Oxide Minerals
What accounts for the complexity and impressive variety of
Mn oxide minerals? Mn occurs in natural systems in three
different oxidation states: +2, +3, and +4, giving rise to a
range of multivalent phases. Mn oxides also display a remark-
able diversity of atomic architectures, many of which easily
accommodate a wide assortment of other metal cations.
Finally, Mn is abundant in most geological systems and forms
minerals under a wide range of chemical and temperature
conditions, and through biological interactions.
Most Mn oxide minerals are brown-black and typically occur
as intimately intermixed, fine-grained, poorly crystalline
masses or coatings. Not surprisingly, identifying the particular
mineral~s) in a Mn oxide specimen can pose quite a challenge.
Hence many scientists report simply "Mn oxide," rather than
a particular mineral phase. Geologists have attempted to avoid
the problem by simply referring to all soft (i.e., it blackens your
fingers), brown-black, fine-grained specimens that were as-
sumed to be Mn oxides as "wad." Similarly, hard (does not
OCR for page 3449
Colloquium Paper: Post
blacken your fingers), gray-black, botroyoidal, massive speci-
mens were called "psilomelane." Recent studies have shown
that most so-called psilomelane specimens are predominantly
the mineral romanechite (Ba.66MnsO~O 1.34H2O). There is no
comparable correlation between specimens labeled wad and
any particular Mn oxide mineral.
Even today identification of the minerals in many Mn oxide
samples is not straightforward. In general, powder x-ray dif-
fraction is diagnostic for well-crystallized, monophasic sam-
ples. Unfortunately, the crystal structures, and consequently,
the powder diffraction patterns are similar for many of the Mn
oxide minerals. In many cases, it is necessary to supplement
powder x-ray diffraction studies with other techniques, such as
transmission electron microscopy (TEM), IR spectroscopy,
and electron microprobe analysis.
Despite the fact that Mn oxides have been extensively
studied for the past several decades, the details of many of their
atomic structures are poorly understood, and there are several
phases for which even the basic crystal structures are not
known. This paucity of crystallographic data has greatly hin-
dered research on the fundamental geochemical behaviors of
common Mn oxide minerals. In most cases the major limiting
factor is the lack of crystals suitable for single-crystal x-ray or
neutron diffraction experiments. In the past few years, how-
ever, other techniques such as TEM (high-resolution imaging
and electron diffraction) and Rietveld refinements using pow-
der x-ray and neutron diffraction data have provided impor-
tant new insights into the atomic structures of Mn oxide
minerals. The Rietveld method (see review in ref. 47) has made
it possible to partially solve and refine structures from data
collected from even relatively poorly crystalline samples (e.g.,
refs. 48 and 49~. Other methods that have contributed to the
understanding of Mn-oxide atomic structures include: IR
spectroscopy, extended x-ray absorption fine-structure spec-
troscopy, and thermogravimetric analysis. The recent appli-
cation of charge-coupled device imaging plates to single-
crystal x-ray diffraction experiments, particularly at synchro-
tron sources (50), should open possibilities for detailed studies
of extremely tiny crystals of certain Mn oxide phases that were
too small for use in conventional experiments.
The basic building block for most of the Mn oxide atomic
structures is the MnO6 octahedron. These octahedra can be
assembled by sharing edges and/or corners into a large variety
of different structural arrangements, most of which fall into
one of two major groups: (i) chain, or tunnel, structures and (ii)
layer structures. The tunnel Mn oxides are constructed of
single, double, or triple chains of edge-sharing MnO6 octahe-
dra, and the chains share corners with each other to produce
frameworks that have tunnels with square or rectangular cross
sections. The larger tunnels are partially filled with water
molecules and/or cations. The layer Mn oxides, sometimes
referred to as phyllomanganates, consist of stacks of sheets, or
layers, of edge-sharing MnO6 octahedra. The interlayer re-
gions can host water molecules and a wide range of cations.
One of the complexities of Mn oxide crystal chemistry is the
multiple valence states exhibited by Mn, commonly even in a
single mineral. It is reasonably straightforward to measure the
average Mn oxidation state for a mineral, but it is considerably
more difficult to determine the proportions of Mn(IV),
Mn(III), and/or Mn(II). In some cases, chemical analyses with
bulk oxidation state measurements unambiguously indicate
the correct Mn valence state, e.g., pyrolusite tMn(IV)02] or
manganosite [Mn(II)O]. For other minerals, detailed crystal
structure studies were able to reveal the valence state infor-
mation. Manganite (MnOOH), for example, can be charge-
balanced assuming all Mn(III) or a half-and-half mixture of
Mn(II) and Mn(IV). Crystal structure refinements revealed
that the Mn in manganite is in a highly distorted octahedral
site, characteristic of the Jahn-Teller effects displayed by
Mn(III). The Mn-O bond distances determined by crystal
Proc. Natl. Acad. Sci. USA 96 (1999) 3449
structure refinements also can provide insights into the Mn
valence state~s), e.g., structure studies of hollandite minerals
(51), romanechite (52), and todorokite (48) indicate that the
reduced form of Mn in these minerals is Mn(III). Studies of
Mn oxidation states also have been performed by using x-ray
spectroscopy (e.g., ref. 53) and x-ray absorption near-edge
structure spectroscopy (54~.
Summarized below are descriptions of the atomic structures
and other information for some of the most important Mn
oxide minerals (for a more in-depth presentation see ref. 55~.
I~he minerals and their chemical formulae are listed in Table 1.
Mn Oxide Minerals with Tunnel Structures
Pyrolusite MnO2. There are three known mineral poly-
morphs of MnO2; pyrolusite is the most stable and abundant,
the others are ramsdellite and nsutite. In pyrolusite (~3-MnO2),
single chains of edge-sharing Mn(IV)06 octahedra share
corners with neighboring chains to form a framework structure
containing tunnels with square cross sections that are one
octahedron by one octahedron (lX1) on a side (Fig. 1~. The
structure is analogous to that of rutile (TiO2~. The chain
structure manifests itself in pyrolusite's typically acicular crys-
tal morphology. The tunnels in pyrolusite are too small to
accommodate other chemical species, and chemical analyses
indicate that the composition deviates at most only slightly
from pure MnO2. Pyrolusite commonly occurs as low-
temperature hydrothermal deposits or as replacements after
other Mn oxide minerals, particularly ramsdellite and manga-
nite. It has long been assumed that Mn oxide dendrites, and
other coatings, commonly found on rock surfaces are pyro-
lusite, but IR spectroscopy studies (70) have revealed that most
such deposits are birnessite and/or romanechite, and in no
case was pyrolusite identified.
Ramsdellite MnO2. In the ramsdellite structure the
Mn(IV)06 octahedra are linked into double chains, each of
which consists of two adjacent single chains that share octa-
hedral edges. The double chains, in turn, link corners with each
other to form a framework having tunnels with rectangular-
shaped cross sections that are 1X2 octahedra on a side (57)
(Fig. 1). The tunnels are generally empty but chemical analyses
commonly reveal minor amounts of water, Na, and Ca that
presumably are located in the channels. Ramsdellite is a
relatively rare mineral, usually occurring in low-temperature
hydrothermal deposits and commonly associated with, and
Table 1. Important Mn oxide minerals
Mineral
Chemical formula Reference
MnO2
MnO2
Mnl:O,OH)2
Bax(Mn4+,Mn3+~0~,6
Kx(Mn4+,Mn3+~so~6
Nax(Mn4+,Mn3+~0~.6
Pbx(Mn4+,Mn3+~so~6
Ba.66(Mn4+,Mn3+150Io 1.34H2O
(Ca,Na,Kjx(Mn4+,Mn3+ )60~2 3.5H20
LiAl2(Mn2 + Mn3 + ~ 06~0H i6
ZnMn3O7 3H20
(Na,Ca)Mn70~4 2.8H20
MnO2 nH20
MnOOH
MnOOH
MnOOH
Mn2+Mn3+04
Mn203
Mn(OH)2
MnO
Pyrolusite
Ramsdellite
Nsutite
Hollandite
Cryptomelane
Manjiroite
Coronadite
Romanechite
Todorokite
Lithiophorite
Chalcophanite
Birnessite
Vernadite
Manganite
Groutite
Feitknechtite
Hausmannite
Bixbyite
Pyrochroite
Manganosite
56
57
58
51
51
59
60
52
48
61
62
49
63
64
65
66
67
68
66
69
OCR for page 3450
3450 Colloquium Paper: Post
-
_`
-
_.d
_
1_
Hi_
it'
~ _
r _
1 - _
___
ma_
_I
1 _ - 1
1 _
A
it_
-
~ i
_
-
C
-
r ~
-
~_
L
L
_
- _ ~
__
__
~..~i
L
1
C
-
FIG. 1. Polyhedral representations of the crystal structures of (A)
todorokite, looking approximately parallel to the Mn octahedral chains.
probably altering to, pyrolusite. Ramsdellite is isostructural
with goethite (FeOOH) and gibbsite (AlOOH).
Nsutite MnO2. Nsutite (^y-MnO2) is an important cathodic
material for use in dry-cell batteries. It is named after the large
deposits of the mineral near Nsuta, Ghana. Although classified
as a mineral, nsutite is actually an intergrowth between
pyrolusite and ramsdellite. TEM images reveal a disordered
structure consisting of regions of a ramsdellite-like phase and
areas that appear to be ordered intergrowths of pyrolusite and
ramsdellite (71~. The generally accepted structure model for
nsutite is an alternating intergrowth of ramsdellite and pyro-
lusite (Fig. 1), and therefore, most so-called nsutite samples
are, in fact, mixtures of ramsdellite and nsutite. TEM studies
Proc. Natl. Acad. Sci. USA 96 (1999'
B
J
1
~_
~_ ~
~_ .
__
_ ~
_ ~
l
1
E
pyrolusite, (B) ramsdellite, (C) hollandite, (D) romanechite, and (E)
also revealed some tunnels larger than lX1 (pyrolusite) and
1X2 (ramsdellite), e.g., 1X3 and 3X3, as well as numerous
defects and grain boundaries (71). All of these complexities
undoubtedly affect the chemical and electrical properties of
the material. Chemical analyses of nsutite typically show minor
amounts of Na, Ca, Mg, K, Zn, Ni, Fe, Al, and Si, and about
2-4 weight percent water (58). These species probably are
accommodated in the larger tunnels or along grain boundaries,
and charge balance is maintained by substituting Mn(III) for
some of the Mn(IV).
Nsutite has been found in ore deposits worldwide (58), and
one occurrence has been reported in ocean Mn nodules (72).
It is a secondary, replacement mineral that commonly forms
from oxidation of Mn carbonate minerals (58).
OCR for page 3451
Colloquium Paper: Post
Hollandite Group R.~_~.sEMn(IV),Mn(III)]x0~6 R = Ba, Pb,
K or Na. As in ramsdellite, the hollandite structure is con-
structed of double chains of edge-sharing MnO6 octahedra, but
they are linked in such a way as to form tunnels with square
cross sections, measuring two octahedra on a side (Fig. 1~. The
tunnels are partially filled with large uni- or divalent cations
and, in some cases, water molecules. The charges on the tunnel
cations are balanced by substitution of lower valence cations
te.g., Mn(III), Fe(III), Al(III), etc.] for some of the Mn(IV).
The different minerals in the hollandite group are defined on
the basis of the predominant tunnel cation: hollandite (Ba),
cryptomelane (K), coronadite (Pb), and manjiroite (Na).
Natural specimens having end-member compositions are un-
usual, and chemical analyses show a wide range of tunnel
cation compositions. Hollandite minerals commonly occur
intermixed and, in some cases, grade from one to another
along a single crystal. They can be major phases in the oxidized
zones of Mn deposits and important ores. Consistent with their
chain structure, they typically are found as fibrous crystals,
usually in compact botroyoidal masses. Less commonly, hol-
landite minerals form as prismatic crystals in hydrothermal
vein deposits.
In recent years there has been considerable interest in the
hollandite minerals and in the hollandite structure-type in
general, both for potential applications as solid ionic conduc-
tors (73) and for immobilizing certain radioactive cations as
part of a waste storage system (74~. Also it has been shown that
at high pressures feldspar minerals transform to a hollandite-
like structure (75), making this, perhaps, an important struc-
ture type in the lower crust and upper mantle.
Romanechite Ba.66Mn(IV)3.6~Mn(III)~.320~0 1.34H2O. The
romanechite structure is constructed of double and triple
chains of edge-sharing MnO6 octahedra that link to form large
tunnels with rectangular cross sections, measuring two by three
octahedra (Fig. 1~. The tunnels are filled with Ba cations and
water molecules in a 1:2 ratio, and the charges on the tunnel
cations are balanced by substitution of Mn(III) for some of the
Mn(IV). Single-crystal x-ray diffraction studies indicate that
the trivalent Mn concentrates on the octahedral sites that are
at the edges of the triple chains. Romanechite typically occurs
as botroyoidal masses in oxidized zones of Mn-rich deposits.
Single crystals, at least those large enough for x-ray diffraction
studies, are known only from Schneeberg, Germany. Cross
sections of botroyoidal samples typically show very fine con-
centric layering (layers are tens to hundreds of microns thick).
Electron microprobe analyses reveal minor fluctuations in
composition among the different layers, mostly in concentra-
tion of Ba relative to Na, K, Ca, and Sr. In general, chemical
analyses of romanechite deviate only slightly from the ideal
formula.
TEM studies have shown that romanechite and hollandite
commonly intergrow on a very fine scale (76~. The two
structures interconnect via the common double octahedral
chain. Upon heating above about 550°C, romanechite trans-
forms to hollandite (77~.
Todorokite (Ca, Na, K).3.5[Mn(IV), Mn(III), Mg]6O~23-
4.5H2O. Todorokite is one of the major Mn minerals identified
in ocean Mn nodules (10 A manganate), and the likely host
phase for strategic metals such as Ni, Co, etc. It is also a major
mineral in the oxidized zones of many terrestrial Mn deposits.
For many years the crystal structure of todorokite was a subject
of considerable conjecture and controversy. Todorokite occurs
with apparent platy or fibrous morphologies, supporting ar-
guments for a tunnel- or layer-type structure (18~. High-
resolution TEM images (77) confirmed that todorokite has a
tunnel structure constructed of triple chains of MnO~ octahe-
dra. The triple chains share corners with each other to form
large tunnels with square cross sections that measure three
octahedra on a side (Fig. 1~. TEM images also revealed
Proc. Natl. Acad. Sci. USA 96 (1999J 3451
intergrowths with todorokite having tunnels measuring 3X4,
3X5, and up to 3X9 octahedra in cross section (78, 79~.
The crystal structure of todorokite recently was refined by
using the Rietveld method and powder x-ray diffraction data,
revealing for the first time the major water and cation positions
in the tunnels (48~. Lower valence cations such as Mn(III),
Ni(II), and Mg(II), which substitute for Mn(IV) to offset
charges on the tunnel cations, appear to be concentrated into
the sites at the edges of the triple chains, as in romanechite.
Chemical analyses of todorokite show considerable variation
in tunnel cation composition (80), and samples from ocean
nodules have up to several weight percent Ni, Co, and/or Cu
(81~. Because of todorokite's large zeolite-like tunnels, there
has been considerable interest in recent years in producing
synthetic analogues for possible use as catalysts or molecular
sieves (82~.
Todorokite typically occurs in Mn deposits as an alteration
product of primary ores such a braunite. It also seems to be a
important phase in many Mn coatings, dendrites, and varnishes
(70~. In the case of ocean nodules, the mechanism of todoro-
kite formation is not well understood, but some experiments
suggest that biological processes might play an important role
(83~. It has been speculated that nodular todorokite alters
from a precursor buserite-like phase (48~. Recently, studies
have shown that todorokite can be synthesized starting with a
Mg-rich birnessite-like phase (45~.
MnOOH Minerals. There are three natural polymorphs of
MnOOH; manganite is the most stable and abundant, the
other two are feitknechtite and groutite. The manganite
(^y-MnOOH) crystal structure is similar to that of pyrolusite
but all of the Mn is trivalent and one-half of the O atoms are
replaced by hydroxyl anions. The Mn(III) octahedra are quite
distorted because of Jahn-Teller effects. Manganite typically
occurs in hydrothermal vein deposits as acicular or prismatic
crystals, or as an alteration product of other Mn-bearing
minerals. In air manganite alters at 300°C to pyrolusite (84),
and many crystals that appear to be manganite, in fact, are
pseudomorphs, having been replaced by pyrolusite.
Groutite (cx-MnOOH) is isostructural with ramsdellite, but,
as in manganite, with all Mn(III) and one-half of the O anions
replaced by hydroxyl anions. Groutite is not a common min-
eral, but sometimes is intimately mixed with pyrolusite, to
which it is probably altering.
In 1945 a hydrous Mn oxide was synthesized that yielded an
x-ray diffraction pattern identical to that of hausmannite with
the exception of a strong extra reflection (d = 4.62 ii) and a
general weakening of the remaining reflections, and it was
given the name hydrohausmannite (85~. Later a mineral was
described from Franklin, NJ that gave an identical x-ray
diffraction pattern to that of hydrohausmannite (86~. Even-
tually it was determined that the original hydrohausmannite
actually was a mixture of two phases: hausmannite and
13-MnOOH (87~. Electron micrographs showed that the
,B-MnOOH crystallized as hexagonal plates. It was assumed
that all so-called hydrohausmannite mineral specimens were
also mixtures, and the name feitknechtite was proposed for
naturally occurring ,8-MnOOH (66~. Feitknechtite is known
only in very fine-grained mixtures, and consequently, its crystal
structure has not yet been determined.
Mn Oxide Minerals with Layer Structures
Lithiophorite LiAI2 [Mn(IV)2Mn(III)] 06(OH)6. The lithio-
phorite structure consists of a stack of sheets of MnO6
octahedra alternating with sheets of A1(OH)6 octahedra in
which one-third of the octahedral sites is vacant (Fig. 2~. In the
ideal formula, Li cations fill the vacant sites in the A1 layer, and
charge balance is maintained by substitution of an equal
number of Mn(III) for Mn(IV) cations (61~. The layers are
cross-linked by H bonds between hydroxyl H on the A1/Li layer
OCR for page 3452
3452 Colloquium Paper: Post
Proc. Natl. Acad. Sci. USA 96 (1999J
5:~
_~
_. ~
c
FIG. 2. Polyhedral representation of (A) lithiophorite showing alternately stacked layers of MnO6 (blue) and (Al, Li)(OH)c (red) octahedra,
(B) Chalcophanite with Zn cations (green octahedra) occupying positions above and below vacancies in the Mn octahedral layers, and (C) Na-rich
birnessite-like phase showing disordered H20/Na sites (yellow) sandwiched between the Mn octahedral sheets.
and O atoms in the Mn sheet. Lithiophorite commonly is found
in weathered zones of Mn deposits and in certain acid soils, but
also has been reported from low-temperature hydrothermal veins
(884. It typically occurs in finely crystalline masses, but in Post-
masburg, South Africa is found as large (1-2 cm) hexagonal
plates. Chemical analyses show that the Li content of lithiophorite
ranges from 0.2 to 3 weight percent and that transition metals
such as Ni, Cu. and Co commonly substitute into the structure
(89, 90~. Extended x-ray absorption fine-structure spectroscopy
suggest that Ni and Cu concentrate in the A1-OH sheets and Co
in the Mn layers (91~. Materials known as asbolanes are important
components of certain Mn ore deposits and are thought to have
a crystal structures similar to that of lithiophorite but with Al
replaced by transition metal cations (92~.
Chalcophanite ZnMn3073H2O. Chalcophanite is a com-
mon weathering product in many Mn-bearing base metal
deposits. Its structure consists of sheets of edge-sharing
Mn(IV)O6 octahedra that alternate with layers of Zn cations
and water molecules (Fig. 2~. One of seven octahedral sites in
the Mn layer is vacant, and the Zn cations are above and below
the vacancies (62~. The water molecules form a hexagonal
close-packed layer with one of seven molecules absent. Min-
erals recently have been described with the same structure as
Chalcophanite but with Mg (93) and Ni (94) instead of the Zn
cations. The crystal structure of Chalcophanite has been of
interest because it is similar to, and, therefore, can serve as a
model for, that of the more abundant and environmentally
important birnessite, which has not been found in crystals
suitable for detailed structural studies.
Birnessite Group [Na,Ca,Mn(II)]Mn7O~42.8H2O. Birnes-
site was first described as a natural phase from Birness,
Scotland (95), and since then it has been recognized that
birnessite and birnessite-like minerals occur in a wide variety
of geological settings. As was mentioned above, it is a major
phase in many soils and an important component in desert
varnishes and other coatings and in ocean Mn nodules. It is
also commonly found as an alteration product in Mn-rich ore
deposits. It readily participates in oxidation-reduction and
cation-exchange reactions and therefore might play a signifi-
cant role in soil and groundwater chemistry.
All known natural birnessite samples are fine-grained and
relatively poorly crystalline. Consequently, it has not been
possible to perform detailed studies of the crystal structures of
these materials. The crystal structures of synthetic Na-, K-, and
Mg-birnessite-like phases, however, recently were determined
by using TEM and powder x-ray diffraction (49~. The study
confirmed that the basic structural unit is a sheet of MnO6
octahedra and revealed that the interlayer cations and water
molecules occupied different positions in each of the three
phases (Fig. 2~. Powder x-ray diffraction patterns of the
minerals ranceite and takanelite suggest that they are isostruc-
tural with birnessite, but with the dominant interlayer cations
being Ca and Mn(II), respectively.
The reported cation-exchange capacity of a synthetic Na-
birnessite-like phase is 240 meq/100 g, with a preference for
Ni and Ba over Ca and Mg (45~.
"Buserite." When Na-birnessite is prepared synthetically, the
initial precipitate yields a powder x-ray diffraction pattern similar
to that of birnessite but with a 10 ~ interlayer spacing, which upon
drying collapses to the typical 7 A birnessite spacing. The collapse
presumably involves the loss of a water layer and is irreversible.
The natural phase that is the presumed analogue of the synthetic
10 ~ material has been called buserite (not an approved mineral
name) (96-98) and might be a common component of ocean Mn
nodules before they dry out (99, 100~. Cations such as Ni(II),
Mg(II), Ca(II), and Co(II) tend to stabilize the buserite structure
against collapse (101, 102~.
Vernadite MnO2 nH2O. Vernadite is a fine-grained poorly
crystalline natural Mn oxide phase characterized by a powder
x-ray diffraction Pattern with broad XRD lines at 2.46, 1.42,
and rarely at 2.2 A. Vernadite appears to be analogous to the
synthetic phase 8-MnO2. Chemical analyses of Vernadite sam-
ples commonly show minor amounts of K, Mg, Ca, Ba, and Fe
(63) and 15-25 weight percent water. The crystal structure of
Vernadite is not known, but it has been proposed that Vernadite
is a variety of birnessite that is disordered in the layer-stacking
direction (103, 104), thereby accounting for the absence of a
basal reflection in the x-ray diffraction pattern. The lack of a
basal reflection also could be the result of individual Vernadite
crystallites that are extremely thin plates, perhaps less than 100
A thick, such that there is no Bragg diffraction arising from the
stacking direction. (66, 103~. Vernadite is found in the oxidized
zone of Mn ore deposits and might be a major phase in ocean
Mn nodules and other Mn oxide crusts and coating.
Other Mn Oxide Minerals
Hausmannite tMn(II)Mn(III)204] has a spinal-like structure
with Mn(II) in the tetrahedral and Mn(III) in the octahedral
OCR for page 3453
Colloquium Paper: Post
sites. It and Bixbyite t(Mn,Fe)203] typically are found in
hydrothermal or metamorphic deposits. The amount of Fe that
can be accommodated into the b~xbyite structure is a function
of temperature (105), and therefore the mineral is an impor-
tant geothermometer in some ore deposits. The crystal struc-
ture of pyrochroite tMn(OH)2] consists of stacked sheets of
Mn(II)(OH)6 octahedra, and manganosite (MnO) is isostruc-
tural with halite. Both minerals are relatively rare, typically
occurring in low-temperature hydrothermal veins in Mn-rich
deposits.
Summary
There are more than 30 Mn oxide/hydroxide minerals, and
many of them occur abundantly in a wide variety of geological
settings. In addition to being important as ores of Mn metal,
they also play an active role in the environmental geochemistry
at the Earth's surface. Mn oxides are ubiquitous in soils and
sediments, and because they are highly chemically active and
strong scavengers of heavy metals, they exert considerable
influences on the compositions and chemical behaviors of the
sediments and soils and associated aqueous systems. The
ability to successfully model and predict the chemical and
thermodynamic properties of Mn oxide minerals and to pre-
pare synthetic analogues depends to a large degree on a
detailed understanding of their crystal structures. Unfortu-
nately, many Mn oxide minerals occur only as fine-grained,
poorly crystalline aggregates and coatings, making crystal
structure studies extremely challenging. In recent years, how-
ever, an arsenal of new techniques, such as TEM, Rietveld
refinements using powder diffraction data, extended x-ray
absorption fine-structure spectroscopy, and single-crystal
studies using charge-coupled device detectors and synchrotron
sources, have slowly started and are continuing to unravel
many of the inner secrets of Mn oxide minerals. Given the
already considerable interest in Mn oxide minerals by geolo-
gists, soil scientists, microbiologists, chemical and environmen-
tal engineers, ceramicists, etc., the future for this group of
characteristically dark minerals looks very bright.
12.
1. Turekian, K. K. & Wedepohl, K. L. (1961) Geol. Soc. Am. Bull.
72, 175-192.
2. Crerar, D. A., Cormick, R. K. & Barnes, H. L. (1980) in Geology
and Geochemistry of Manganese, eds. Varentsov, I. M. & Gras-
selly, Gy. (E. Schweizerbart'sche Verlagsbuchhandlung, Stutt-
gart), Vol. 1, pp. 293-334.
3. Varentsov, I. M. (1996) Manganese Ores of Supergene Zone:
Geochemistry of Formation (Kluwer, Boston).
4. Roy, S. (1997) in Manganese Mineralization: Geochemistry and
Mineralogy of Terrestrial and Marine Deposits, eds. Nicholson, K.,
Hein, J. R., Buhn, B. & Dasgupta, S. (Geological Society,
London), Spec. Pub. 119, pp. 5-27.
5. Lankford, W. T., Jr., Samways, N. L., Craven, R. F. & McG-
annon, H. E., eds. (1984) The Making, Shaping, and Treating of
Steel (Assoc. of Iron and Steel Engineers, Pittsburgh, PA).
6. U.S. Geological Survey (1998) Mineral Industry Surveys: Man-
ganese (U.S. Geological Survey, Reston, VA).
7. Crerar, D. A. & Barnes, H. L. (1974) Geochim. Cosmochim.
Acta 38, 279-300.
8. Murray, J. & Renard, A. F. ( 1891) Report on the Scientific Results
of the Exploration Voyage of H.M.S. Challenger (Neill and Co.,
Longon).
9. Menard, H. W. & Shipek, C. J. (1958) Nature (London) 182,
1156-1158.
10. Riley, J. P. & Sinhaseni, P. (1958) J. Mar. Res. 17, 466-482.
Mero, J. L. (1977) in Marine Manganese Nodules, ed. Glasby,
G. P. (Elsevier, Amsterdam), pp. 327-355.
Horn, D. R., Horn, B. M. & Delach, M. N. (1972) in Ferro
manganese Deposits on the Ocean Floor, ed. Horn, D. R.
(Lamont-Doherty Observatory, Columbia Univ., New York),
pp. 9-17.
13. Cronan, D. S. & Tooms, J. S. (1967) Deep-Sea Res. 14, 117-119.
Proc. Natl. Acad. Sci. USA 96 (1999J 3453
14. Ku, T. L. & Glasby, G. P. (1972) Geochim. Cosmochim. Acta 36,
699-703.
15. Crecelius, E. A., Carpenter, R. & Merrill, R. T. (1973) Earth
Planet. Sci. Lett. 17, 391-396.
16. Rona, P. A., Harrison, R. N., Bassinger, B. G., Scott, R. B. &
Nalwalk, A. J. (1976) Geol. Soc. Am. Bull. 87, 661-674.
17. Hein, J. R., Koschinsky, A., Halbach, P., Manheim, F. T., Bau,
M., Kang, J. & Lubick, N. (1997) in Manganese Mineralization:
Geochemistry and Mineralogy of Terrestrial and Marine Deposits,
eds. Nicholson, K., Hein, J. R., Buhn, B. & Dasgupta, S.
(Geological Society, London), Spec. Pub. 119, pp. 123-138.
18. Burns, R. G. & Burns, V. M. (1977) in Marine Manganese
Nodules, ed. Glasby, G. P. (Elsevier, Amsterdam), pp. 185-248.
19. Barnes, S. S. (1967) Science 157, 63-65.
20. Glasby, G. P. (1972) Mar. Geol. 13, 57-72.
21. Weisz, P. B. (1968) J. Catal. 10, 407-408.
22. Wu, S. & Chu, C. (1972) Atmos. Environ. 6, 309-317.
23. Ehrlich, H. L. (1972) in Ferromanganese Deposits on the Ocean
Floor, ed. Horn, D. R. (Lamont-Doherty Observatory, Colum-
bia Univ., New York), pp. 63-70.
24. Tebo, B. M., Ghiorse, W. C., van Waasbergen, L. G., Siering,
P. L. & Caspi, R. (1997) in Geomicrobiology: Interactions
Between Microbes and Minerals, eds. Banfield, J. F. & Nealson,
K. H. (Mineral. Soc. Am., Washington, DC), Rev. Min. 35, pp.
225-266.
Chao T. T. & Theobald, P. K. (1976) Econ. Geol. 71, 1560-1569.
Nicholson, K. & Eley, M. (1997) in Manganese Mineralization:
Geochemistry and Mineralogy of Terrestrial and Marine Deposits,
eds. Nicholson, K., Hein, J. R., Buhn, B. & Dasgupta, S.
(Geological Society, London), Spec. Pub. 119, pp. 309-326.
27. Murray, J. W. (1975) Geochim. Cosmochim. Acta 39, 505-519.
28. Jenne, E. A. (1968) in Trace Inorganics in Water (Am. Chem.
Soc., Washington, DC), ACS Advances in Chemistry Series,
Vol. 73, pp. 337-387.
29. Young, L. B. & Harvey, H. (1992) Geochim. Cosmochim. Acta
56, 1175-1186.
30. Yao, W. & Millero, F. J. (1996) Environ. Sci. Technol. 30,
536-541.
31. Carpenter, R. H., Robinson, G. D. & Hayes, W. B. (1978) J.
Geochem. Explor. 10, 75-89.
32. Whitney, P. R. (1975) J. Geochem. Explor. 4, 251-263.
33. Prasad, V. S. & Chaudhuri, M. (1995) Aqua 44, 80-82.
34. Lind, C. J. & Hem, J. H. (1993) Appl. Geochem. 8, 67-80.
35. Prusty, B. G., Sahu, K. C. & Godgul, G. (1994) Chem. Geol. 112,
275-291.
36. Towler, P. H., Smith, J. D. & Dixon, D. R. (1996) Anal. Chim.
Acta 328, 53-59.
37. Todd, J. F., Elsinger, R. J. & Moore, W. S. (1988) Mar. Chem.
23, 393-415.
38. Wei, C. L. & Murray, J. W. (1991) Deep-Sea Res. 38, 855-873.
39. Taylor, R. M., McKenzie, R. M. & Norrish, K. (1964) Austr. J.
Soil Res. 2, 235-248.
40. Golden, D. C., Dixon, J. B. & Kanehiro, Y. (1993)Austr. J. Soil
Res. 31, 51-66.
41. Scott, M. J. & Morgan, J. J. (1996) Environ. Sci. Technol. 18,
807-815.
42. Manceau, A. & Charlet, L. (1992) J. Colloid Interface Sci. 148,
425-442.
43. Huang, P. M. (1991) in Rates of Soil Chemistry Processes, eds.
Sparks, D. L. & Suarez, D. L. (Soil Sci. Soc. Am., Madison, WI),
pp. 191-230.
44. Driehaus, W., Seith, R. & Jekel, M. (1995) Water Res. 29,
297-305.
45. Golden, D. C., Chen, C. C. & Dixon, J. B. (1986) Science 231,
717-719.
46. Morgan, J. J. & Stumm, W. (1965) Adv. Water Pollut. Res. 1,
103-118.
47. Post, J. E. & Bish, D. L. (1989) in Modern Powder Diffiaction,
eds. Bish, D. L. & Post, J. E. (Mineral. Soc. of Am., Washington,
DC), Rev. in Mineralogy 20, pp. 277-308.
48. Post, J. E. & Bish, D. L. (1988) Am. Mineral. 73, 861-869.
49. Post, J. E. & Veblen, D. R. (1990) Am. Mineral. 75, 477-489.
50. Pluth, J. J., Smith, J. V., Pushcharovsky, D. Y., Semenov, E. I.,
Bram, A., Riekel, C., Weber, H. P. & Broach, R. W. (1997) Proc.
Natl. Acad. Sci. USA 94, 12263-12267.
Post, J. E., Von Dreele, R. B. & Buseck, P. R. (1982) Acta
Crystallogr. B 38, 1056-1065.
OCR for page 3454
3454 Colloquium Paper: Post
52. Turner, S. & Post, J. E. (1988) Am. Mineral. 73, 1155-1161.
53. Yanchuk, E. A. (1977) Lithol. Miner. Res. 12, 733-737.
54. Schulze, D. G., Sutton, S. R. & Bait, S. (1995) Soil Sci. Soc.
Am. J. 59, 1540-1548.
55. Post, J. E. (1992) in Biomineralization, Processes of Iron and
Manganese, eds. Skinner, H. C. W. & Fitzpatrick, R. W. (Catena
Verlag, Cremlingen-Destedt, Germany), pp. 51-73.
56. Baur, W. H. (1976) Acta Crystallogr. B 32, 2200-2204.
57. Bystrom, A. M. (1949) Acta Chem. Scand. 3, 163-173.
58. Zwicker, W. K., Meijer, W. O. J. G. & Jaffe, H. W. (1962)Am.
Mineral. 47, 246-266.
59. Nambu, M. & Tanida, K. (1967) J. Jpn. Assoc. Mineral. Petrol.
Econ. Geol. 58, 39-54.
60. Post, J. E. & Bish, D. L. (1989) Am. Mineral. 74, 913-917.
61. Post, J. E. & Appleman, D. E. (1994)Am. Mineral. 79, 370-374.
62. Post, J. E. & Appleman, D. E. (1988) Am. Mineral. 73, 1401
1404.
63. Chukhrov, F. V., Gorshkov, A. I., Rudnitskaya, E. J., Ber
ezovskaya, V. V. & Sivtsov, A. V. (1978) Izv. Akad. Nauk SSSR'
Ser. Khim. 6, 5-19.
64. Dachs, H. (1963) Zeit. Kristall. 118, 303-326.
65. Glasser, L. S. D. & Ingram, L. (1968) Acta Crystallogr. B 24,
1233-1236.
66. Bricker, O. (1965) Am. Mineral. 50, 1296-1354.
67. Satomi, K. (1961) J. Phys. Soc. Jpn. 16, 258-265.
68. Geller, S. (1971) Acta Crystallogr. B 27, 821-828.
69. Sasaki, S., Fujino, K., Takeuchi, Y. & Sadanaga, R. (1980)Acta
C'ystallogr. A 36, 904-915.
70. Potter, R. M. & Rossman, G. R. (1979J Am. Mineral. 64,
1219-1226.
71. Turner, S. (1982) Ph.D thesis (Arizona State Univ., Tempe).
72. Manheim, F. T. (1965) in Symposia on Marine Chemistry, eds.
Schink, D. R. & Corless, J. T. (Occasional Publications, Univ.
Of Rhode Island, Kingston), Vol. 3, 217-276.
73. Beyeler, H. U. (1976) Phys. Rev. Lett. 37, 1557-1560.
74. Ringwood, A. E., Kesson, S. E., Ware, N. G., Hibberson, W. &
Major, A. (1979) Nature (London) 278, 219-223.
75. Ringwood, A. E. & Reid, A. F. (1967) Acta Crystallogr. 23,
1093-1099.
76. Turner, S. & Buseck, P. R. (1979) Science 203, 456-458.
77. Fleischer, M. & Richmond, W. E. (1943) Econ. Geol. 38,
269-286.
78. Turner, S. & Buseck, P. R. (1981) Science 212, 1024-1027.
79. Chukhrov, F. V., Gorshkov, A. I., Sivtsov, A. V. & Beresovskaya,
V. V. (1978) Izv. Akad. Nauk SSSR, Ser. Khim. 12, 86-95.
Proc. Natl. Acad. Sci. USA 96 (1999)
101.
102.
103.
80. Ostwald, J. (1986) Mineral. Mag. 50, 336-340.
81. Burns, V. M. & Burns, R. G. (1978) Earth Planet. Sci Lett. 39,
341-348.
82. Shen, Y. F., Zerger, R. P., Deguzman, R. N., Suib, S. L.,
McCurdy, L., Potter, D. I. & O'Young, C. L. (1993) Science 260,
511-515.
83. Mandernack, K. W., Post, J. E. & Tebo, B. M. (1995) Geochim.
Cosmochim. Acta 59, 4393-4408.
84. Dasgupta, D. R. (1965) Mineral. Mag. 35, 131-139.
85. Feitknecht, W. & Marti, W. (1945) Helv. Chim. Acta 28,
129-148.
86. Frondel, C. (1953) Am. Mineral. 38, 761-769.
87. Feitknecht, W., Brunner, P. & Oswald, H. R. (1962) Z. Anorg.
Allg. Chem. 316,154-160.
88. De Villiers, J. E. (1983) Econ. Geol. 78, 1108-1118.
89. Ostwald, J. (1984) Mineral. Mag. 48, 383-388.
90. Ostwald, J. (1984) Ore Geol. Rev. 4, 3-45.
91. Manceau, A., Llorca, S. & Calas, G. (1987) Geochim. Cosmo-
chim. Acta 51, 105-113.
Chukhrov, F. V., Gorshkov, A. I., Vitovskaya, V. I., Drits, V. A.,
Sivtsov, A. V. & Rudnitskaya, Y. S. (1982) Int. Geol. Rev. 24,
598-604.
Yan, G., Zhang, S., Zhao, M., Ding, J. & Li, D. (1992) Acta
Mineral. Sinica 121, 69-77.
94. Grice, J. D., Gartrell, B., Gault, R. A. & Van Velthuizen, J.
(1994) Can. Mineral. 32, 333-337.
95. Jones, L. J. P. & Milne, A. A. (1956) Mineral. Mag. 31, 283-288.
96. Giovanoli, R., Feitknecht, W. & Fischer, F. (1971) Helv. Chim.
Acta 54, 1112-1124.
97. Burns, R. G., Burns, V. M. & Stockman, H. W. (1983) Am.
Mineral. 68, 972-980.
98. Giovanoli, R. (1985) Am. Mineral. 70, 202-204.
99. Arrhenius, G. O. & Tsai, A. G. (1981) Scripps Inst. Oceanogr.
81-28, 1-19.
100. Ostwald, J. & Dubrawski, J. V. (1987) Neues Jahr. Mineral.
Monatsh. 157, 19-34.
Giovanoli, R. & Burki, P. (1975) Chimia 29,114-117.
Paterson, E., Clark, D. R., Russell, J. D. & Swaffield, R. (1986)
Clay Minerals 21, 957-964.
Giovanoli, R. & Arrhenius, G. (1988) in The Manganese Nodule
Belt of the Pacific Ocean, eds. Halbach, P., Friedrich, G. & von
Stackelberg, U. (Ferdinand Enke Verlag, Stuttgart), pp. 20-37.
104. Giovanoli, R. (1980) Miner. Deposita 15, 251-253.
105. Mason, B. (1944) Am. Mineral. 29, 66-69.
Representative terms from entire chapter:
crystal structure
