Oxides & Hydroxides

Anion for oxides

O^2-

Anion for hydroxides

OH^-

General properties for oxides and hydroxides

High melting point, low specific gravity

Oxide classification

Simple oxides, multiple oxides, hydroxides

Oxide sub-groups

Hematite group, spinel group, rutile group

Hydroxide sub-groups

Aluminum hydroxides, iron hydroxides

Simple oxide classification

1 cation with O^-2

Multiple oxide classification

>1 cation with O^-2

Hydroxide classification

Contains OH^-, O directly bound to a metal in a single bond

Structural properties of hematite

Isostructural with corundum, Fe³⁺ in octahedral coordination

Chemical compound for hematite

Fe₂O₃

General properties of hematite

Semi-metallic luster as a crystal, black color, cherry red to brown streak

Places of occurrence for hematite

Magmatic + hydrothermal deposits, deposits in quartz veins, skarns, and quartzite, occurs in earthy masses called “red iron ore”

Structural properties for illmenite

Isostructural with corundum and hematite, but cations are ordered in alternate layers parallel to the close-packed O^-2 atom; tabular/platy crystals

Chemical formula for illmenite

FeTiO₃

General properties for illmenite

Metallic luster, black color, minimal magnetism

Places of occurrence for illmentite

Occurs in segregations, veins, and layers in pyrossenite and gabbro.

Structural properties of corundum

Hexagonal crystal system, euhedral crystals, steep hexagonal bipyramidal faces

Chemical formula for corundum

Al₂O₃

Chemical properties for corundum

Strong ionic-covalent bonds, often contains Cr, Ti, Fe

General properties for corundum

Vitreous luster, high density, blue with Fe+Ti present (sapphire), red with Cr present (ruby), yellow when Fe alone is present, can never exist with quartz due to the chemical reaction

Places of occurrence for corundum

Hydrothermal alterations of volcanic and ultra mafic rocks under regional metamorphism of Al-rich clays and alkaline magma

General formula for spinel group minerals

XY₂O₄

General formula for hematite group minerals

X₂O₃

Structural properties of spinel

Cubic symmetry

Chemical formula of spinel

MgAl₂O₄

Chemical properties of spinel

Interstices are occupied by Al³⁺ and Mg²⁺ (Mg usually occupies smaller tetrahedral spaces, isomorphic substitutions of Fe²⁺, Fe³⁺, Mn²⁺, Cr²⁺, Zn²⁺

General properties of spinel

Vitreous luster, substitutions cause color (normally colorless), Cr = pink, Fe = green or green-black, high hardness (8, diagnostic)

Places of occurrence for spinel

Metamorphic settings, high temperature and pressure, associated with calcite, diopside, phlogopite

Chemical formula of magnetite

Fe²⁺Fe³⁺₂O₄

Chemical properties of magnetite

Iron can vary greatly, when subject to weathering it oxidizes and causes hematite and goethite to form

General properties of magnetite

Black color, high hardness (6), magnetic

Places of occurrence for magnetite

Deposits in peridotite, gabbro, and skarns

General chemical formula for rutile group minerals

XO₂

Structural properties of rutile

Octahedral coordination (CN: 6) edges of octahedral are shared forming chains parallel to the c-axis —> chains are cross linked at octahedral corners, tetragonal symmetry, prismatic and acicular crystals, crystals have well developed tetragonal-bipyramindal faces, good {110} cleavage

Chemical formula for rutile

TiO₂

General properties of rutile

NOT FIBROUS, black, pale-yellow, reddish brown, adamantine luster, high hardness (6.5)

Places of occurrence for rutile

Quartz veins and metamorphic rocks

Structural properties of pyrolusite

Isostructural with rutile, occurs in earthy masses + oolites

Chemical compound for pyrolusite

MnO₂

General properties for pyrolusite

Metallic to dull luster, black color, black streak

Places of occurrence for pyrolusite

Layers and lenses in sedimentary rocks and earthy aggregates in oxidation zones

Chemical properties of hydroxides

Ionic sheets are electrostatic neutral, held together by weak van der Waals bonds —> perfect planar cleavage

Structural properties of brucite

Fine grained masses + sometimes parallel acicular aggregates

Chemical formula for brucite

Mg(OH)₂

General properties for brucite

Rare

Places of occurrence for brucite

Crystallizes as a product of hydrothermal alteration of ultramafic rocks and some marbles, replacing olivine

Structural properties of gibbsite

Perfect (001) cleavage

Chemical formula for gibbsite

Al(OH)₃

General properties for gibbsite

Rarely forms crystals

Places of occurrence for gibbsite

Occurs together with other products of hydrothermal alteration

Structural properties of geothite

Orthorhombic symmetry, compact fine grained aggregates, rare individual acicular or fibrous habit

Chemical formula of geothite

α-Fe³⁺O(OH)

General properties of goethite

Dull luster, rusty-goldish-brown color, rusty brown streak

Places of occurrence of goethite

Occurs as weathering or alteration of hematite and other oxidized Fe-bearing minerals

Structural properties of lepidochrocite

Orthorhombic symmetry, platy of acicular habit, double ribbons of distorted Fe(O,OH)₆ octahedra (linked at corners)

Chemical formula of lepidochrocite

γ-Fe³⁺O(OH)

General properties of lepidochrocite

Dull luster, gold-brown, rusty brown streak

Places of occurrence of lepidochrocite

{not listed}