Chapter 3 — Minerals

Mineral

A homogeneous, naturally occurring, solid inorganic substance with a definable chemical composition and an orderly internal atomic arrangement.

Mineral class

A group of minerals distinguished on the basis of chemical composition.

Crystal lattice

The orderly framework in which the atoms or ions of a mineral are fixed.

Why glass is not a mineral

Glass has no atomic order / crystal lattice.

Crystal

A single continuous piece of a mineral bounded by flat surfaces that formed naturally as it grew.

Crystal face

A flat surface that grew flat as a crystal grew.

Crystal structure

The arrangement and packing of atoms in a mineral; it depends on chemical composition.

Ionic bond

Bond formed by attraction between oppositely charged ions after electron transfer.

Halite bonding example

Na transfers an electron to Cl, forming Na+ and Cl-; their attraction forms ionic NaCl (halite).

Polymorphs

Two minerals with the same chemical composition but different crystal lattice structures.

Diamond and graphite

Both are carbon polymorphs; their different crystal structures produce very different physical properties.

Covalent bond

Bond formed when uncharged atoms share electrons in their outermost orbits.

Diamond hardness

Diamond is hard because of its ordered crystal structure and strong covalent bonds between carbon atoms.

Graphite softness

Graphite is soft because of its crystal structure even though it is made of the same element, carbon, as diamond.

Mineral identification properties

Color, streak, luster, hardness, specific gravity, crystal habit, cleavage/fracture, and other properties such as acid reaction or magnetism.

Color

A sometimes-useful mineral property, but the same mineral can occur in many colors; the lecture calls it the poorest identification tool.

Why quartz can have different colors

Minute impurities can change quartz color; titanium can produce rose quartz and iron can make amethyst purple.

Streak

The color of a mineral in powdered form; a useful mineral-identification property.

Luster

The way a mineral surface scatters light.

Metallic luster

Luster that looks metal-like; pyrite is the lecture example.

Nonmetallic luster

Luster that does not look metallic; feldspar is the lecture example.

Hardness

Measurement of a mineral's bonds resisting breaking; stronger crystal bonds generally mean a harder mineral.

Mohs hardness scale

Relative hardness scale from 1 to 10.

Mohs 1

Talc.

Mohs 2

Gypsum.

Mohs 3

Calcite.

Mohs 4

Fluorite.

Mohs 5

Apatite.

Mohs 6

Orthoclase.

Mohs 7

Quartz.

Mohs 8

Topaz.

Mohs 9

Corundum.

Mohs 10

Diamond.

Fingernail hardness benchmark

About 2.5 on the Mohs scale.

Copper penny hardness benchmark

About 3.5 on the Mohs scale.

Wire nail hardness benchmark

About 4.5 on the Mohs scale.

Glass / knife blade hardness benchmark

About 5.5 on the Mohs scale.

Streak plate hardness benchmark

About 6.5 on the Mohs scale.

Specific gravity

A number representing mineral density: the ratio of the weight of a mineral volume to the weight of an equal volume of water.

Galena specific gravity

Very high specific gravity, about 7.4-7.6.

Ulexite specific gravity

Very low specific gravity, about 1.955.

Crystal habit

The shape of a mineral's crystal faces, or lack of developed crystal faces.

Cleavage

Tendency of a mineral to break along preferred planes controlled by atomic bonds.

Mica cleavage

Mica has good cleavage in one direction.

Halite cleavage

Breaks into cubes with 90-degree angles between faces.

Calcite cleavage

Breaks into rhombohedra with about 120-degree and 70-degree angles.

Cleavage vs crystal faces

Cleavage can look blocky or step-like; crystal faces tend to be smooth or may show striations.

Fracture

Irregular mineral breakage; garnet and quartz are examples.

Quartz fracture

Broken quartz fractures like glass into irregular, sharp fragments.

Calcite acid reaction

Weak HCl can break the bond holding Ca to the carbonate group in calcite.

Lodestone

Naturally magnetic magnetite.

Cation

Atom or group of atoms with an overall positive charge due to loss of an electron.

Anion

Atom or group of atoms with an overall negative charge due to gain of an electron.

Oxides

Mineral class with metal cations bonded to oxygen anions; magnetite (Fe3O4) is an example.

Sulfides

Mineral class with metal cations bonded to sulfide anions; pyrite (FeS2) is an example.

Sulfates

Mineral class with metal cations bonded to a sulfate anionic radical; gypsum is the lecture example.

Halides

Mineral class in which a cation is bonded to a halogen ion such as Cl or F; halite (NaCl) is an example.

Hydroxides

Mineral class with cations bonded to an OH anionic radical; brucite Mg(OH)2 is an example.

Phosphates

Mineral class with cations bonded to a PO4 anionic radical; apatite is an example.

Carbonates

Mineral class with a cation bonded to a CO3 anionic radical; calcite CaCO3 is an example.

Native elements

Pure masses of a single element; gold (Au) is an example.

Silicates

Mineral class with a cation bonded to the SiO4 anionic radical; olivine is an example.

Calcite bonding

Calcite contains covalent bonds within carbonate groups and ionic attraction between Ca2+ and CO3^2-.

Silicon-oxygen tetrahedron

Building block of all silicate minerals: one Si covalently bonded to four O atoms.

Independent silicate tetrahedra

Olivine is the lecture example.

Single-chain silicates

Pyroxene is the lecture example.

Double-chain silicates

Amphibole is the lecture example.

Sheet silicates

Mica is the lecture example.

3-D silicate framework

Silicon-oxygen tetrahedra can link in three dimensions by sharing oxygen.