Oxide and hydroxide minerals
Mineral phases containing only oxide or
hydroxide anions in their structures. By volume, oxide and hydroxide minerals
comprise only a small fraction of the Earth's crust. However, their geochemical
and petrologic importance cannot be overstated. Oxide and hydroxide minerals are
important ores of metals such as iron, aluminum, titanium, uranium, and
manganese. Oxide and hydroxide minerals occur in all geological environments.
Some form as primary minerals in igneous rocks, while others form as secondary
phases during the weathering and alteration of silicate and sulfide minerals.
Some oxide and hydroxide minerals are biogenic; for example, iron(III) and
manganese(IV) hydroxides and oxides often result from bacterial oxidation of
dissolved Fe2+ and Mn2+ in low-temperature aqueous solutions. See also: Hydroxide; Mineral;
Iron and manganese hydroxide minerals often occur as nanocrystalline or colloidal phases with high, reactive surface areas. Adsorption of dissolved aqueous ions onto colloidal iron and manganese oxides plays a major role in the fate of micronutrients and heavy metals in soil and ground water and the trace-element chemistry of the oceans. Much current research is focused on measuring the thermodynamics and kinetics of metal adsorption by colloidal Fe-Mn hydroxides and oxides in the laboratory. In anoxic sedimentary environments, bacteria may use iron(III) and manganese(IV) hydroxide minerals as electron acceptors. Consequently, these minerals may facilitate the biodegradation of organic pollutants in soil and ground water. See also: Adsorption; Colloid
Bonding and crystal chemistry
To a first approximation, the bonding in
oxide minerals can be viewed in the ionic (electrostatic) model. According to
Pauling's rules, the coordination number of a metal cation (such as Mg2+, Al3+,
and Ti4+) is determined by the radius of the cation relative to that of the
oxide anion (O2−). This allows one to predict the structures of some of the
simpler oxide minerals (Table 1). Cations with similar ionic radii (such as Mg2+
and Fe2+) are able to substitute for each other and form a solid solution.
Sulfide minerals (which are more covalent) show little solid solution. The
simple ionic radii arguments will fail when electronic configurations preclude
the spherical symmetry of the cations. The ions Cu2+ and Mn3+, with nine and
four d electrons, tend to adopt distorted coordination environments because of
the Jahn-Teller effect. Also, d-electron configurations can give rise to large
octahedral site-preference energies that cause small cations, such as Mn4+, to
always adopt octahedral coordination. The magnetic and semiconducting properties
of transition-metal oxide minerals, such as magnetite (Fe3O4), give useful
geophysical signatures for subsurface exploration. See also: Ionic crystals; Jahn-Teller
effect; Prospecting; Solid-state chemistry; Structural chemistry;
Survey of minerals
The following is a summary of the important oxide and hydroxide minerals by structural classification.
The mineral cuprite (Cu2O) forms during the low-temperature oxidation of primary copper sulfide minerals. It is sometimes an important ore of copper. In the structure of cuprite, Cu+ ions are in twofold coordination with oxygen. No other monovalent cations form stable X2O oxide minerals. See also: Copper
Oxides of divalent metals, such as Fe2+ and
Mg2+, with the formula XO will adopt the NaCl structure (Fig. 1) in which the
metal cation is in sixfold coordination. Fe2+ and Mg2+ have nearly identical
ionic radii, and a phase made up of the solid solution (Mg,Fe)O, called
ferropericlase, is probably the second most abundant mineral in the Earth's
lower mantle. Experimental evidence indicates that it maintains the NaCl
structure over the pressure range of the Earth's interior. The end member MgO
(periclase) is quite rare as a crustal mineral; it occurs in some metamorphosed
limestones. Most other XO oxide minerals (such as manganosite MnO and bunsunite
NiO) are also very rare. Not all XO oxides adopt the NaCl structure. Because the
ionic radius of Zn2+ is somewhat smaller than that of Mg2+, the mineral zincite
ZnO has a structure based on the tetrahedral coordination of Zn. The mineral
tenorite (CuO) is a secondary alteration phase of copper sulfides and has a
structure based on the square planar coordination of Cu2+. This structure
results from the Jahn-Teller distortion of the CuO6 coordination polyhedron.
Larger divalent cations, such as Sr and Ba, form oxide structures based on
eightfold coordination, but these are not stable minerals. See also:
Fig. 1 NaCl structure adopted by XO oxides such as MgO. Small spheres are divalent cations such as Mg2+. Large spheres are O2− anions.
X2O3 oxides and ilmenite
Trivalent cations, such as Fe3+ and Al3+, having radii appropriate for sixfold-coordination with oxygen, will adopt the corundum (α-Al2O3) structure (Fig. 2). Hematite (α-Fe2O3) is a common phase in soils and sediments and forms by the oxidation of Fe2+ in primary silicates. (Rust is mostly hematite.) Hematite is the most important ore mineral of iron and is the dominant mineral found in Precambrian banded iron formations. These vast deposits formed because of the oxidation of dissolved Fe2+ in the oceans when the Earth's atmosphere accumulated oxygen from photosynthetic bacteria. Corundum is a minor accessory mineral in metamorphic rocks and occurs in peraluminous igneous rocks. Partial solid solution is found between corundum and hematite. The gemstone ruby is Al2O3 with minor Cr3+, while sapphire is Al2O3 with other chromophores. A modification of the corundum structure is found in the mineral ilmenite (FeTiO3). This is an important accessory mineral in felsic igneous rocks. Above 950°C (1740°F), there is complete solid solution between hematite and ilmenite. Bixbyite (Mn2O3) is a distorted corundum structure resulting from the Jahn-Teller effect in Mn3+. See also: Banded iron formation; Igneous rocks; Iron; Metamorphic rocks; Ruby; Sapphire
Fig. 2 Polyhedral representation of Al2O3 structure adopted by X2O3 oxides. Each octahedron represents an M3+ cation surrounded by six O2− anions, which define the vertices of the octahedral.
Tetravalent cations, such as Ti4+, Sn4+, and
Mn4+, whose ionic radii favor sixfold coordination, adopt the rutile structure
(Fig. 3). Rutile (TiO2) is a common accessory mineral in felsic igneous rocks,
gneisses, and schists. It also has two low-temperature polymorphs, anatase and
brookite, but these are less common. Cassiterite (SnO2) is the only significant
ore mineral of tin. Cassiterite occurs mostly in granite-hosted hydrothermal
deposits such as those in
Fig. 3 Rutile structure adopted by XO2 oxides.
Large tetravalent cations, such as U4+ and Th4+, prefer to be in eightfold coordination with oxygen and form oxides with the fluorite structure (Fig. 4). Uraninite (UO2) is the most important ore of uranium and is a primary mineral in granites. See also: Radioactive minerals; Thorium; Uranium
Fig. 4 Fluorite structure adopted by uraninite (UO2) and thorianite (ThO2).
The spinel structure (Fig. 5) is adopted by oxides with the formula X2+Y3+2O4. The spinel structure has one tetrahedral cation site and two octahedral cation sites per four oxygens. In a normal spinel, the tetrahedral site is occupied by a divalent cation such as Mg2+, Fe4+, while the octahedral sites are occupied by trivalent cations such as Fe3+, Cr3+, or Al3+. The inverse spinel structure is a variation where the tetrahedral sites are occupied by trivalent cations and the octahedral sites are occupied by a mixture of divalent and trivalent cations. A variety of solid solutions are possible within the spinel structure oxides.
Fig. 5 Spinel structure adopted by XY2O4 oxides. In the normal spinel structure (for example, MgAl2O4), tetrahedral sites are occupied by divalent cations such as Mg2+ and octahedral sites are occupied by trivalent cations such as Al3+. The inverse-spinel structure (for example, Fe3O4) has trivalent cations in the octahedral sites and a mixture of divalent and trivalent cations in the octoctahedral sites.
The most important spinel structure oxide is magnetite (Fe3O4). Magnetite is an inverse spinel, so half of the Fe3+ cations are in the tetrahedral sites and the remaining Fe3+ cations, along with the Fe2+ cations, are in the octahedral sites. Electron hopping between Fe2+ and Fe3+ cations in the octahedral sites gives magnetite a high electrical conductivity. The most important geophysical property of magnetite is its ferrimagnetism, with a Néel temperature, the temperature at which an antiferromagnetic material becomes paramagnetic, of 525°C (980°F). As an igneous rock cools, the magnetic moments of individual magnetite domains align with the Earth's magnetic field. This preserves a record of the orientation of the rock relative to the Earth's magnetic field at the time of crystallization. These paleomagnetic signatures in rocks were used to confirm the hypothesis of sea-floor spreading and continental drift. Magnetite often contains significant amounts of other cations such as Cr3+ and Ti4+. A complete solid solution between Fe3O4 and Fe2TiO4 (ulvospinel) is stable above 600°C (1100°F). See also: Antiferromagnetism; Chromium; Ferrimagnetism; Geomagnetism; Magnetic susceptibility; Paleomagnetism; Titanium
The structure of hausmannite (Mn3O4), is a distortion of the spinel structure because of the Jahn-Teller effect for octahedrally coordinated Mn3+. Hausmannite is found in high-temperature hydrothermal veins and in metamorphosed sedimentary manganese deposits but is not very common.
The spinel oxide, chromite (FeCr2O4), is the dominant ore mineral of Cr. Chromite occurs in ultramafic rocks and in the serpentinites that are derived from them; significant ore deposits are found in Iran and Zimbabwe. Because of the high octahedral site preference energy of Cr3+, chromite has a normal spinel structure. See also: Coordination chemistry; Serpentinite
Manganese(III, IV) oxides and oxide hydroxides
Manganese hydroxides and oxides containing Mn4+ and Mn3+ form a variety of structures based on chains, tunnels, and sheets of MnO6 polyhedra (Table 2). The variations in the Mn oxidation state give variations in the charge of the MnO6 sheets and tunnel/chain frameworks. The layer and framework charges are compensated by the incorporation of cations (such as K+, Ba2+, and Pb2+) in the interlayer and tunnel sites. Perhaps the most important example is birnessite (Fig. 6) which is a mixed-valence Mn4+-Mn3+ layer-structured oxide. This mineral, and the related phase vernadite (“δ-MnO2”; probably an incoherently stratified birnessite), are major phases in marine ferromanganese nodules and crusts which form on the sea floor. At least two structural modifications are present for birnessite due to the presence of cation vacancies in the sheets; ordering of the vacancies can lower the symmetry of the sheets from hexagonal to triclinic. In the interlayer, cations, such as Li, Al, and Zn2+, will adsorb above the vacancy sites to give structures such as lithiophorite (Li,Al)(Mn4+,Mn3+)O2(OH)2 and chalcophanite (Li, Al)(Mn4+, Mn3+)O2(OH)2. The former occurs in manganese nodules formed in acid soils. Chalcophanite is much less common and forms in the oxidized zone of Zn-Mn ore deposits (such as the much-studied Franklin and Stirling Hill locality in New Jersey). See also: Manganese nodules
Fig. 6 Structure of birnessite. Between the MnO2 layers are large exchangeable hydrated cations such as K+, Ca2+, Na+.
The simplest tunnel structure (Fig. 7) is based on double chains of MnO6 polyhedra; this is adopted by hollandite and related phases with formula A0-2(Mn4+, Mn3+)8(O,OH)16 (A = Ba, K, Pb, Na) [Table 2]. Todorokite, also found in marine manganese crusts and nodules, is a tunnel structure based on treble chains of MnO6 polyhedra (Fig. 8). Incorporation of ions into manganese oxides must have important controls on the trace-element chemistry of the oceans.
Fig. 7 The 2 × 2 tunnel structure adopted by hollandite and related manganese(IV, III) oxides. The tunnels can accommodate cations such as Ba2+, K+, Na+, and Pb2+. The same structure, but with Cal− anions in the tunnel sites, is adopted by akaganeite (β-FeOOH).
Fig. 8 Structure of todorokite. Within the 3 × 3 tunnels, exchangeable hydrated cations such as K+, Mg2+, and Ba2+ are present.
The simplest hydroxide mineral is brucite [Mg(OH)2], the structure of which is based on Mg(OH)2 layers that are held together by hydrogen bonds. Brucite forms during the alteration of magnesium silicates by hydrothermal fluids. It is not very common. The brucite structure also is adopted by hydroxides of other divalent cations such as Ca2+, Fe2+, and Ni2+. The end member Fe(OH)2 is easily partially oxidized, and the charge balance is maintained by the incorporation of various anions (such as CO2−3, SO2−4, and Cl−) between the (Fe2+, Fe3+)(OH)2 layers to give “green rusts.” These minerals are probably important phases in subsonic and anoxic soils and sediments. The mineral gibbsite [Al(OH)3] has a structure based on layers of Al(OH)6 octahedral with the layers held together by hydrogen bonds. See also: Hydrogen bond
MOOH oxide hydroxides and related minerals
Trivalent cations, such as Fe3+ and Al3+, form several oxide hydroxide structures. These minerals usually occur as clay-sized (<2 μm) particles in soils and sediments. Colloidal particles of oxide hydroxide minerals also are suspended in most natural waters. The surfaces of these minerals are quite reactive and strongly adsorb ions from aqueous solutions. In the environment, the aqueous concentrations of many trace micronutrients and toxic heavy metals are probably controlled by adsorption onto iron oxide hydroxide mineral surfaces. The most common FeOOH phases are goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) [Figs. 9 and 10]. Goethite tends to form by hydrolysis of dissolved Fe3+, while lepidocrocite forms by oxidation of green rust. The aluminum analogues, diaspore (α-AlOOH) and boehmite (γ-AlOOH), along with gibbsite (Al(OH)3) are the minerals that make up bauxite, the ore of Al formed by weathering of primary silicate minerals such as feldspar in tropical soils. There is only limited solid solution between the FeOOH and AlOOH (or MnOOH) phases. Several other FeOOH minerals are known, but they are less common. Akaganeite (“β-FeOOH”) forms by the hydrolysis of Fe3+ in chloride-bearing solutions and is a minor phase in marine sediments. Its structure is similar to that of hollandite but the tunnels are occupied by Cl− anions. The mineral schwertmannite is believed to be similar to akaganeite, but with SO2−4 anions occupying the tunnels. This mineral forms in acid mine drainage, probably by bacterially mediated oxidation of dissolved Fe2+.
Fig. 9 Structure of goethite (α-FeOOH) and diaspore (α-AlOOH). The MnO2 polymorph ramsdellite (α-MnO2) also adopts this structure.
Fig. 10 Structure of lepidocrocite (γ-FeOOH) and boehmite (γ-AlOOH).
Perhaps the most ubiquitous FeOOH type mineral is ferrihydrite. This phase is poorly crystalline and forms by the rapid hydrolysis of dissolved Fe3+. This is facilitated by bacterial oxidation of dissolved Fe2+ under less acidic conditions than those favoring schwertmannite. With time, ferrihydrite dissolves and recrystallizes to form the more stable phases goethite and hematite. Nevertheless, the extremely high, reactive surface area of ferrihydrite (up to 600 m2/gram) means that it can have a strong effect on aqueous geochemistry by sorbing dissolved ions.
David M. Sherman
D. H. Lindsey (ed.), Oxide Minerals, Mineralogical Society of America, 1991
J. E. Post, Manganese oxide minerals: Crystal structures and economic and environmental significance, Proc, Nat. Acad. Sci., 96:3447–3454, 1999
U. Schwertmann and R. M. Cornell, Iron Oxides in the Laboratory: Preparation and Characterization, 2d ed., Wiley-VCH, Weinheim, 2000
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