GEOL 215 | Ch.3 & 3.5 - Crystallography & Formation Properties

Ionic substitution

The process of one ion replacing another

Example of complete substitution

Olivine formula: (Mg, Fe)SiO4

Forsterite end member: (Mg)SiO4

Fayalite end member: (Fe)SiO4

Coupled Ionic substitution

The process of simultaneous replacement of ions with two different charges in two difference sites that preserves electrical neutrality of the crystal lattice

Coupled Ionic substitution example

Plagioclase formula: (Na, Ca)(Si, Al)AlSi2O8

Charge imbalance occurs when Ca+2 and Na+1 substitute

Balanced by 2nd substitution involving Si+4 and Al +3.

Limited substitution

When ions of substantially different sizes substitute

Limited substitution example

MgCO3 and CaCO3

• Calcite (Ca)

• Magnesite (Mg)

Ca ions are 30% large than Mg, limited substitution between the two end members

Some compositions do not exist in nature and form a miscibility gap

Phase stability diagrams

Display a stability field of certain minerals (area which X mineral is stable)

The Phase Rule

Governs the number of phases that coexist in equilibrium in any system

P = C + 2 - F

P = number of phases present (composition, structure, and/or state)

C = components (e.g., Mg, Fe, Si, and O, in olivine)

F = Degrees of Freedom (variance) number of independent variables (e.g., pressure and temp)

Crystallography

Long range order or crystal structure of crystals

Crystal faces

Formed when minerals grow, and enclose crystalline solids when they stop growing

Simple symmetry operations

Repetition of a unit of pattern or motif using a single type of operation

• Compound symmetry = using two operations

Key symmetry operations

1. Translation

2. Rotation

3. Reflection

4. Inversion

Compound symmetry operations

1. Glide reflection

2. Rotoinversion

3. Screw rotation

Glide rotation

Combination of mirror and translation symmetry

Rotoinversion

Combination of rotation about an axis and inversion

Screw Axes

Combination of rotation and translation

Primitive (P)

They have nodes only at their corners and have a content of one node (one motif)

Non-primitive (C)

Contain extra nodes in one or more faces or centers and posses more that one node or motif

Unit Cell

Smallest unit within a mesh that contains at least one node. Must fit together without any gaps.

Twinning

Two or more adjacent crystals of the same mineral are oriented so that they share some of the same crystal lattice points in a symmetrical manner

How do we actually form minerals?

1. Precipitation from solution

   • Ground or surface water

   • Hydrothermal fluids

2. Sublimation from gases

3. Crystallization from a melt

   • Lava or magma bodies

4. Solid state growth

5. Solid-liquid or solid-gas exchange

   • Weathering, hydrothermal alteration

Granular

Subequant, macroscopic crystal aggregate with a granular appearance; as in marble

Lamellar, Foliated, Micaceous

As in mica

Fibrous

As in asbestos

Acicular (needlelike) and radiating

Acicular-filiform crystals radiating outward from a central point; as in millerite

Bladed

As in actinolite

Dendritic

Tree-like, branching network; as in pyrolusite

Botryoidal

Rounded, mound-like aggregates; as in hematite

Colloform and Stalactitic

As in cave deposties

Macroscopic Mineral Properties

Hardness, Density & Specific Gravity, Tenacity, Crystal Faces, Effervescence, Luster, Streak, Color, Luminescence, and Magnetism

Hardness

Assessed using the Mohs hardness scale

Range from 1-10 (H)

Copper Coin ~3.5, fingernail ~2.5, steel nail ~6.5
Talc 1, Gypsum 2, Corundum 9, Diamond 10

Density (p)

Mass per unit volume of a material

Specific Gravity (SG)

Ratio between the density of the material and the density of water

Tenacity

The manner in which a mineral responds to short term stresses

Malleable, Ductile, Brittle

Crystal Faces

When a crystal stops growing it might not have perfect geometric growth faces, 3 terms can describe how “perfect” the face is;

Euhedral, Subhedral, and Anhedral

Euhedral

Perfect faces, flat and geometric

Subhedral

Some faces but not perfect

Anhedral

No crystal faces visible

Cleavage

Some minerals break in distinct patterns

180/basal = e.g, biotite, muscovite, mica

90/90 or 60/120 = e.g., pyroxene, amphibole, feldspar

Opacity

Opaque or transparent

Luster

The appearance of the mineral surface under reflected light

Idiochromatic

“self-colored”

Allochromatic

“foreign colored” - influenced strongly by impurities

Streak

The color of the mineral powder

Magnetism

Response to an external magnetic field

Dimagnetic minerals

Not attracted to even very powerful magnets

Paramagnetic minerals

Weakly attracted to strong magnets, lose magnetization when external field is removed

(e.g., Olivine, pyroxene, biotite)

Ferromagnetic-ferrimagnetic minerals

Strongly attracted to even weak magnets and can retain magnetization for long periods of time

(e.g., Magnetite, titanomagnetite, pyrrhotite)