Mineralogy: Crystal Systems, Mineral Groups, and Formation Methods

A comprehensive vocabulary review of crystal systems, mineral structures, oxide, hydroxide, and phosphate mineral specimens, and various mineral formation and alteration processes.

Trigonal crystal system

A crystal system characterized by having $3$-way rotational symmetry.

Hexagonal crystal system

A crystal system characterized by having $6$-way rotational symmetry.

Mineral habit

The external shape of a mineral based on its internal structure, where the shape of a larger crystal tends to share symmetry with the unit cell.

Crystal twinning

A phenomenon that occurs when $2$ adjacent crystals share some of the same points on their crystal lattice by intergrowing with each other during formation.

Pseudomorphs

A 'false form' where a mineral retains its original properties but takes the shape of a different crystal, such as azurite to malachite or calcite to agate.

Oxides

Minerals in which an oxygen ion is bonded with a metal ion; they typically form at higher temperatures.

Hydroxides

Minerals that possess the $OH$ ion in place of some or all oxygen ions; they form at a lower temperature compared to oxides.

GEOTHITE

An iron mineral that is dark brown to black in color with a brownish-yellow streak, basal cleavage, and an often botryoidal or massive habit, formed from the weathering of other iron materials.

LIMONITE

A yellowish-brown mineral that is not crystalline in appearance, looks like coral, and is basically the same in origin as geothite.

HEMATITE

An ore of iron that has a reddish-brown streak, a metallic luster, and forms black or red tabular crystals as well as massive aggregates.

MAGNETITE

The only mineral that is magnetic; characterized by a black streak, a metallic luster, black or gray color, and an octahedral crystal habit.

PROLUSITE

An oxide mineral exhibiting radiating, dark metallic acicular crystal aggregates.

RUTILE

An oxide mineral occurring as slender, elongated prismatic and acicular reddish-brown crystal clusters.

CORUNDUM

An oxide mineral that crystallizes into hexagonal tabular or barrel-shaped forms.

ZINCITE

A pretty rare mineral that is only naturally found in New Jersey.

Phosphates

A mineral class consisting of a metal bonded with a phosphate ion; they primarily form in igneous rocks, provide phosphorus in fertilizers, and most species are uncommon.

TURQUOISE

A phosphate mineral that commonly forms botryoidal, light blue-green crusts.

FLUORAPATITE (APATITE)

A phosphate mineral that forms prominent hexagonal prismatic crystals.

PYROMORPHITE

A phosphate mineral noted for having an appearance that 'looks like fried eggs'.

VANADINITE

A mineral described as 'tabular and pretty like squary and rectangley'.

Hydrothermal mineral formation

The precipitation of minerals (becoming solid out of a solution) at high temperatures, occurring when water heated by magma moves through rock fractures and precipitates minerals as it reaches lower temperature and pressure.

Evaporites

Minerals formed by chemical precipitation from surface saline solutions; as water evaporates and dissolved elements saturate, minerals precipitate out when sufficient water is lost.

Chemical weathering

A process where water, $O_2$, and $CO_2$ react with a rock and change its mineral composition.

Dissolution

A form of chemical weathering in which a mineral dissolves completely.

Hydrolysis

A chemical weathering reaction between a mineral and water.

Oxidation

A chemical weathering reaction involving $O_3$.

Carbonation

A chemical weathering reaction between a mineral and carbonic acid derived from rainwater.

Metamorphism

Alterations driven by high heat and pressure that alter chemical or physical structure, causing partially melted minerals to recrystallize and replacing old minerals with new ones.

Bowen's reaction series

A sequence describing the crystallization order of minerals from magma.