Minerals in geology 3

Uses of minerals in geologyDetermining • Ages of rocks • Tectonic environment • Compositions of source magma • Pressure and temperature histories of rocks • Reaction rates • Past strain

The Amazing World of

Minerals

Photos: www.johnbetts-fineminerals.com

Cueva de los cristales, Naica Mine,

Mexico

• Series of gypsum filled caves found at 950ft depth in a mine

• 122ºF!! 100% humidity!!

• Explorers and scientists must wear refrigerated space suits

to avoid being boiled alive

• Even with the suits they can only remain in the caves for 10 minutes

• Gypsum seems to have formed in unusually saturated geothermal fluids associated with a nearby fault

• Exploration continues today

Photos: La Venta Exploring Team

Why are minerals important?

Short Answer: You can’t live without them!

Bauxite

Aluminum

Halite

Salt

Diamond

Cutting tools, getting married

Feldspar

Ceramics, porcelain

Quartz

Watches, radios, glass

Borax

Soap, cosmetics, fire retardant, fiberglass, fertilizer, insecticide, airplanes,

medicine!

Zeolites

Water purification, catalysts, medicine

Uraninite

Nuclear power, x-rays

Uses of minerals in geology

Determining

• Ages of rocks

• Tectonic environment

• Compositions of source magma

• Pressure and temperature histories of rocks

• Reaction rates

• Past strain recorded in rocks

• Paleomagnetism

• Economic ores

• The chemical make-up of the Earth and how elements are exchanged

Mineral Identification

• Since every mineral is chemically and structurally

unique, every mineral has properties that can be used to distinguish it from other minerals

• A major purpose of this class will be give you the

confidence to identify minerals in the field so you can

use them to answer geological questions

Common Properties for Mineral

Identification

– Ex: Epidote is almost always green

– Ex: Sulfur is almost always yellow

• However, minerals such as quartz, tourmaline and

garnet can be virtually any color

• Most used method is the Mohs Scale

5) Orthoclase 6) Apatite 7) Quartz 8)

Topaz 9) Corundum 10) Diamond

• Minerals with a lower number will be scratched by minerals with a higher number

• Mohs scale is relative (diamond is 10x harder than corundum)

• A description of the way light interacts with the surface of

a mineral or rock

• Luster descriptions include metallic, earthy, waxy,

greasy, glassy, silky, brilliant, dull, satin spar, soapy

Pyrite

metallic

Quartz glassy

Talc Soapy, pearly

Crystal Structure or Habit

• What shape is the crystal?

Bladed Tabular Cube

Dipyramidal Prism Rhombohedron

Also descriptions like fibrous, platy, massive, equant, acicular are helpful

Cleavage and Fracture

• Cleavage occurs along specific planes of weakness in a mineral These planes are caused by the molecular structure of the mineral

• Crystals with good cleavage like calcite or mica will always break parallel to the same plane.

• Number, quality and angular relationships between cleavage planes are important

• Minerals with no cleavage like quartz

will fracture

– Conchoidal or uneven

Density (mass/volume)

• Many minerals leave a characteristic streak color when scratched across a porcelain plate

• Other minerals have no streak

Mineral Assemblages/Tectonic

Environment

• Minerals commonly occur with other

characteristic minerals

– Ex: Scarn minerals: Epidote, Calcite, Garnet,

Scheelite

– Ex: Hydrothermal sulfide deposits: Galena, Barite,

Sphalerite, Pyrite, Fluorite, Calcite

– Ex: Pegmatites: Tourmaline, Quartz, Lepidolite, Beryl, Muscovite, Feldspar

• Some minerals occur in specific environments

– Ex: Zeolite minerals commonly grow in vesicles in

igneous rocks

– Ex: Evaporites commonly occur in desert playas

• Some minerals glow in the presence

of either short or long wave ultraviolet

light There are several minerals that

exhibit this property some of which are

calcite, diamond, fluorite, halite, scheelite

and willemite.

• Fluorescence occurs on the atomic level

in a mineral The electrons of an atom each

have a certain energy level called their 'ground

state' (blue electrons).

• In fluorescent minerals, energy is absorbed by the atom increasing the energy of the electrons, causing them to jump to the next energy level (red electrons).

• This increase in energy level does not last long (approximately 10-8

seconds) When the electrons fall back to their ground state, the extra

energy is emitted from the atom in the form of visible light (green

sparkles).

Selenite

Calcite

Fluorite Calcite with zincite

Diamond

Other Properties used for ID

• Optical Properties

– Ulexite- fiber optic properties

– Calcite- double refraction

– Optical Microscopy

• HCl Acid

– Calcite- fizzes when acid is applied

• Twinning

– Orthoclase feldspar- Carlsbad twinning

– Plagioclase- Albite twinning

Other Properties used for ID

• Magnetism

– Magnetite- magnetic

• Smell

– Sulfur- rotten eggs

• Alteration/Weathering

– Hematite- rusts red

– Olivine- alters to orange mineral called iddingsite

• Taste

– Halite- salt

– If it’s orange/red and you eat it and it kills you it was probably Orpiment/Realgar

Created by Nicolas Barth

2007 Geology 114A University of California, Santa Barbara Some images herein borrowed from websites have not been credited