Minerals and Rocks

Study Aid

Most photos and tables courtesy of Katryn Wiese City College of San Francisco

Minerals(A rock is an aggregation of one or more minerals)

To be a mineral, it will meet all these requirements:

- solid

- naturally occurring

- inorganic

- has a characteristic chemical composition

has a definite internal atomic arrangement (crystal structure).

Thus it will have distinctive physical properties:

-Crystal Habit (Shape)

-Cleavage and Fracture

-Hardness

-Color (often the least useful diagnostic tool)

-Luster (how light plays off the surface)

-Specific Gravity (Density—how heavy it will feel)

-Taste (not recommended since some are toxic)

-Reacts or not to Hydrochloric acid

-Twinning

-Exsolution Lamellae

-Magnetic

-Double Refraction

Using this diagnostic tools in combination will help you identify mineral hand samples in

the field.

double refraction

Sphalerite

Bauxite

Igneous Rocks

Sedimentary Rocks

Metamorphic Rocks

We use mineralogy and texture in a metamorphic rock to determine the agents that

caused the rock to form (the highest temperature and pressure and/or types of fluids to

which a rock was exposed). We call the intensity of metamorphism to which a rock was

exposed metamorphic grade.

GRAPHIC OF BASALT

METAMORPHISM BY

METAMORPHIC SETTING

Textural changes

Major textural changes occur as metamorphic grade increases, due to increased pressures and temperatures. If the

pressures are uniform in all direction (confining pressure), the results are different than if the pressure is high in

only one direction (directed pressure). In the latter case, pressure is released if minerals align themselves

perpendicular to the direction of pressure.

• Density increases (volume shrinks) – Grains/crystals pack closer together under confining pressure.

• Foliation increases – Minerals align when under directed pressure.

• Crystal size increases – Grain boundaries migrate, enlarging crystal size as pressure (any kind) placed on

crystal boundaries.

Other Textural Changes found in metamorphic rocks:

• Veins – Fractures filled by minerals that precipitated from hydrothermal fluids.

• Porphyroblasts – Unusually large crystals set in a finer-grained groundmass.

• Folds, lineations, stretched or sheared grains – Clasts or layers in the original rock are stretched out or

folded under directed pressure.

• Slickensides – Smoothed, grooved surfaces – formed when two rocks move across one another, like along

faults or cracks.

Mineral changes

Minerals can change and grow in metamorphic rocks, without melting. The

chemically active fluids and pressure at crystal grain boundaries can cause the ions

in the solid rock to migrate as though they were in a fluid. For this reason,

metamorphic minerals tend to show some of the most perfect crystal faces. In

addition, as metamorphic grade increases, minerals change to more stable ones

and crystals get larger.

Polymorph example:

These three minerals

have the same chemical

formula, but are stable

at different pressure

and temperature

conditions.

Setting: B=Burial, C=Contact, R=Regional, and S=Subduction.

More Stuff

• Practice Sheets

• Answer Keys

• Flash Cards