oxide mineral, any naturally occurring inorganic compound with a structure based on close-packed oxygen atoms in which smaller, positively charged metal or other ions occur in interstices. Oxides are distinguished from other oxygen-bearing compounds such as the silicates, borates, and carbonates, which have a readily definable group containing oxygen atoms covalently bonded to an atom of another element.
| name | colour | lustre | Mohs hardness | specific gravity |
|---|---|---|---|---|
| anatase | brown to indigo blue and black; also variable | adamantine to metallic adamantine | 5½–6 | 3.8–4.0 |
| boehmite | white, when pure | 3 | 3.0–3.1 | |
| brookite | various browns | metallic adamantine to submetallic | 5½–6 | 4.1–4.2 |
| brucite | white to pale green, gray, or blue | waxy to vitreous | 2½ | 2.4 |
| cassiterite | reddish or yellowish brown to brownish black | adamantine to metallic adamantine, usually splendent | 6–7 | 7.0 |
| chromite | black | metallic | 5½ | 4.5–4.8 |
| chrysoberyl | variable | vitreous | 8½ | 3.6–3.8 |
| columbite | iron black to brownish black; often with iridescent tarnish | 6–6½ | 5.2 (columbite) to 8.0 (tantalite) | |
| corundum | red (ruby); blue (sapphire); also variable | adamantine to vitreous | 9 (a hardness standard) | 4.0–4.1 |
| cuprite | various shades of red | adamantine to earthy | 3½–4 | 6.1 |
| delafossite | black | metallic | 5½ | 5.4–5.5 |
| diaspore | white, grayish white, colourless; variable | brilliant vitreous | 6½–7 | 3.2–3.5 |
| euxenite | black | brilliant submetallic to greasy or vitreous | 5½–6½ | 5.3–5.9 |
| franklinite | brownish black to black | metallic to semimetallic | 5½–6½ | 5.1–5.2 |
| gibbsite | white; grayish, greenish, reddish white | vitreous | 2½–3½ | 2.3–2.4 |
| goethite | blackish brown (crystals); yellowish or reddish brown | adamantine-metallic | 5–5½ | 3.3–4.3 |
| hausmannite | brownish black | submetallic | 5½ | 4.8 |
| hematite | steel gray; dull to bright red | metallic or submetallic to dull | 5–6 | 5.3 |
| ilmenite | iron black | metallic to submetallic | 5–6 | 4.7–4.8 |
| lepidocrocite | ruby red to reddish brown | submetallic | 5 | 4.0–4.1 |
| litharge | red | greasy to dull | 2 | 9.1–9.2 |
| magnetite | black to brownish black | metallic to semimetallic | 5½–6½ | 5.2 |
| manganite | dark steel gray to iron black | submetallic | 4 | 4.3–4.4 |
| massicot | sulfur to orpiment yellow | greasy to dull | 2 | 9.6 |
| periclase | colourless to grayish; also green, yellow, or black | vitreous | 5½–6 | 3.6–3.7 |
| perovskite (often containing rare earths) | black; grayish or brownish black; reddish brown to yellow | adamantine to metallic | 5½ | 4.0–4.3 |
| psilomelane | iron black to dark steel gray | submetallic to dull | 5–6 | 4.7 |
| pyrochlore | brown to black (pyro); pale yellow to brown (micro) | vitreous or resinous | 5–5½ | 4.2–6.4 |
| pyrolusite | light steel gray to iron black | metallic | 2–6 | 4.4–5.0 |
| rutile | reddish brown to red; variable | metallic adamantine | 6–6½ | 4.2–5.5 |
| spinel | various | vitreous | 7½–8 | 3.55 |
| tenorite | steel or iron gray to black | metallic | 3½ | 5.8–6.4 |
| thorianite | dark gray to brownish black and bluish | hornlike to submetallic | 6½ | 9.7–9.9 |
| uraninite | steel to velvet black; grayish, greenish | submetallic to greasy or dull | 5–6 | 6.5–8.5 (massive); 8.0–10.0 (crystals) |
| name | habit | fracture or cleavage | refractive indices or polished section data | crystal system |
| anatase | pyramidal or tabular crystals | two perfect cleavages |
omega = 2.561 epsilon = 2.488 extremely variable |
tetragonal |
| boehmite | disseminated or in pisolitic aggregates | one very good cleavage |
alpha = 1.64–1.65 beta = 1.65–1.66 gamma = 1.65–1.67 |
orthorhombic |
| brookite | only as crystals, usually tabular | subconchoidal to uneven fracture |
alpha = 2.583 beta = 2.585 gamma = 2.700–2.741 |
orthorhombic |
| brucite | tabular crystals; platy aggregates; fibrous or foliated massive | one perfect cleavage |
omega = 1.56–1.59 epsilon = 1.58–1.60 |
hexagonal |
| cassiterite | repeatedly twinned crystals; crusts and concretions | one imperfect cleavage |
omega = 1.984–2.048 epsilon = 2.082–2.140 light gray; strongly anisotropic |
tetragonal |
| chromite | granular to compact massive | no cleavage; uneven fracture |
n = 2.08–2.16 brownish gray-white; isotropic |
isometric |
| chrysoberyl | tabular or prismatic, commonly twinned, crystals | one distinct cleavage |
alpha = 1.746 beta = 1.748 gamma = 1.756 |
orthorhombic |
| columbite | prismatic crystals, often in large groups; massive | one distinct cleavage | brownish gray-white; weakly anisotropic | orthorhombic |
| corundum | pyramidal or barrel-shaped crystals; large blocks; rounded grains | no cleavage; uneven to conchoidal fracture |
omega = 1.767–1.772 epsilon = 1.759–1.763 |
hexagonal |
| cuprite | octahedral, cubic, or capillary crystals; granular or earthy massive | conchoidal to uneven fracture |
n = 2.849 bluish white; anomalously anisotropic and plechroic |
isometric |
| delafossite | tabular crystals; botryoidal crusts | one imperfect cleavage | rosy brown-white; strongly anisotropic; distinctly pleochroic | hexagonal |
| diaspore | thin, platy crystals; scaly massive; disseminated | one perfect cleavage, one less so |
alpha = 1.682–1.706 beta = 1.705–1.725 gamma = 1.730–1.752 |
orthorhombic |
| euxenite | prismatic crystals; massive | conchoidal to subconchoidal fracture | n = 2.06–2.25 | orthorhombic |
| franklinite | octahedral crystals; granular massive |
n = about 2.36 white; isotropic |
isometric | |
| gibbsite | tabular crystals; crusts and coatings; compact earthy | one perfect cleavage |
alpha = 1.56–1.58 beta = 1.56–1.58 gamma = 1.58–1.60 |
monoclinic |
| goethite | prismatic crystals; massive | one perfect cleavage, one less so |
alpha = 2.260–2.275 beta = 2.393–2.409 gamma = 2.398–2.515 gray; strongly anisotropic |
orthorhombic |
| hausmannite | pseudo-octahedral crystals; granular massive | one nearly perfect cleavage |
omega = 2.43–2.48 epsilon = 2.13–2.17 gray-white; distinctly anisotropic |
tetragonal |
| hematite | tabular crystals; rosettes; columnar or fibrous massive; earthy massive; reniform masses | no cleavage |
omega = 2.90–3.22 epsilon = 2.69–2.94 anisotropic; weakly pleochroic; often shows lamellar twinning |
hexagonal |
| ilmenite | thick, tabular crystals; compact massive; grains | no cleavage; conchoidal fracture | n = about 2.7 grayish white; anisotropic | hexagonal |
| lepidocrocite | flattened scales; isolated rounded crystals; massive | one perfect cleavage, one less so |
alpha = 1.94 beta = 2.20 gamma = 2.51 gray-white; strongly anisotropic and pleochroic |
orthorhombic |
| litharge | crusts; alteration product on massicot | one cleavage |
omega = 2.665 epsilon = 2.535 |
tetragonal |
| magnetite | octahedral crystals; granular massive |
n = 2.42 brownish gray; isotropic |
isometric | |
| manganite | prismatic crystals, often in bundles; fibrous massive | one very perfect cleavage, two less so |
alpha = 2.25 beta = 2.25 gamma = 2.53 brownish gray-white; anisotropic; weakly pleochroic |
monoclinic |
| massicot | earthy or scaly massive | two cleavages |
alpha = 2.51 beta = 2.61 gamma = 2.71 |
orthorhombic |
| periclase | irregular, rounded grains; octahedral crystals | one perfect cleavage | n = 1.730–1.746 | isometric |
| perovskite (often containing rare earths) | cubic crystals | uneven to subconchoidal fracture |
n = 2.30–2.38 dark bluish gray |
orthorhombic |
| psilomelane | massive; crusts; stalactites; earthy masses | orthorhombic | ||
| pyrochlore | octahedral crystals; irregular masses | subconchoidal to uneven fracture | n = 1.93–2.02 | isometric |
| pyrolusite | columnar or fibrous massive; coatings and concretions | one perfect cleavage | cream-white; distinctly anisotropic; very weakly pleochroic | tetragonal |
| rutile | slender to capillary prismatic crystals; granular massive; as inclusions, often oriented | one distinct cleavage |
omega = 2.556–2.651 epsilon = 2.829–2.895 |
tetragonal |
| spinel | octahedral crystals; round or embedded grains; granular to compact massive | n = 1.715–1.725 | isometric | |
| tenorite | thin aggregates or laths; curved plates or scales; earthy masses | conchoidal fracture | light gray-white; strongly anisotropic; pleochroic | monoclinic |
| thorianite | rounded cubic crystals | uneven to subconchoidal fracture | n = about 2.2 (variable) isotropic | isometric |
| uraninite | crystals; massive; dendritic aggregates of crystals | uneven to conchoidal fracture | light brownish gray; isotropic | isometric |
The oxide minerals can be grouped as simple oxides and multiple oxides. Simple oxides are a combination of one metal or semimetal and oxygen, whereas multiple oxides have two nonequivalent metal sites. The oxide structures are usually based on cubic or hexagonal close-packing of oxygen atoms with the octahedral or tetrahedral sites (or both) occupied by metal ions; symmetry is typically isometric, hexagonal, tetragonal, or orthorhombic.
The simple oxides can be subdivided on the basis of the ratio of the numbers of atoms of metal (or other elements) and oxygen, giving general formulas of the AxOy type. In such formulas A represents a metal atom, and x and y represent integers. Chemical compositions then fall into categories such as those designated AO, A2O, A2O3, AO2. Specific simple oxide minerals include periclase (MgO), cuprite (Cu2O), hematite (Fe2O3), and uraninite (UO2).
Complex oxides show a more varied chemistry, often with extensive solid solution. Most common is the spinel group, with the general formula AB2O4, in which A and B are ions of different metals, the same metal with different oxidation states, or a combination of the two; A (with oxidation state +2), B (with oxidation state +3) is the commonest, as, for example, in spinel itself, MgAl2O4. Frequently occurring doubly charged ions include magnesium, iron, zinc, and manganese, while common triply charged ions are aluminum, iron, manganese, and chromium.
Oxide minerals occur as decomposition products of sulfide minerals, in pegmatites, early crystallizing minerals in ultrabasic rocks, and as accessory minerals in many igneous rocks.