by Harold L. Levin
Chapter 4 - page 1
Rocks and Minerals: Documents that Record Earth's History
Pamela J. W. Gore
Georgia Perimeter College
Minerals are the building blocks of rocks, and the Earth's crust is made of rocks.
Minerals can provide a wide range of information about past events in Earth's history:
- Minerals may contain radioactive elements that can be used for radiometric age dating.
- Minerals that crystallize from magmas and lavas can provide information about temperatures,
as well as viscosity of the magma, type of volcano, and tectonic setting.
- Minerals that form under metamorphic conditions can provide information about temperatures
and pressures, from which we can determine the depth at which metamorphism occurred, and
information about the history of the formation of mountain ranges.
- Minerals that form by evaporation in arid climates can tell us about paleoclimatic
conditions. Since some climates are controlled by latitude, we can make general
inferences about latitute.
- Minerals that form in sea water tell us about the presence of ancient seas.
- Minerals which contain iron can record the orientation of the Earth's magnetic field,
which yields information on latitude, and provides evidence for drifting continents,
seafloor spreading, and movement and reversal of the Earth's magnetic poles.
- Minerals in sedimentary rocks can provide information on the tectonic setting, amount of relief,
paleoclimate, and types of rocks that are eroding in the source area.
- Minerals can also tell us about the changing chemistry of the atmosphere,
for example, the presence or absence of oxygen.
- Naturally occurring.
- Definite chemical composition.
- Orderly internal crystal structure.
Each mineral has different physical and chemical properties which allow it to be identified.
that can be used to identify minerals include (among others):
- crystal form.
- reaction to acid.
Physical Properties of Minerals
- Color - the color of the mineral as it appears to the eye in reflected light.
Note that some minerals may have a range of colors.
Example: Quartz may be colorless, white, pink, purple, dark brown, green, or blue.
- Streak - the color of a mineral when it is ground to a powder.
Streak color may be quite different from mineral color.
Examples: Hematite may be silver or gray, but it has a reddish brown streak.
Pyrite is golden, but is has a black streak.
- Luster - the character of the light reflected from the mineral.
A mineral may have a metallic luster or a non-metallic luster.
- Hardness - the resistance of a mineral to scratching.
Hardness is measured on a scale of 1 - 10 called Mohs Hardness Scale.
Hardness of minerals can also be compared to common objects (fingernail, copper penny, nail, glass).
The Minerals of
Mohs Hardness Scale
- Talc (softest)
________ penny (copper)
- Orthoclase feldspar
(or potassium feldspar)
- Diamond (hardest)
- Density - how heavy a mineral is for its size.
The mass of a mineral divided by its volume is a measure of its density.
Examples: Quartz has a density of 2.65 g/cm3.
Gold has a higher density of 19.3 g/cm3.
- Crystal form - some minerals are in the form of crystals.
Crystals "grow" through the addition of chemical ions to their surfaces as they crystallize from a
magma or lava, from mineral-rich waters, or from gases. Perfect crystals are rare in nature
because minerals typically grow close together in confined spaces, producing a mass of
interlocking crystals. A crystal which grows in a larger space may develop crystal faces.
The crystal shape is related to the structural arrangement of the atoms within the mineral.
Cleavage - the tendency of a mineral to break along flat surfaces related to planes of weakness
in its crystal structure. Minerals can be identified by the number of cleavage planes they exhibit,
and the angles between them.
Examples: Some minerals tend to cleave or break into flat sheets
(the micas: muscovite and biotite). Others break into cubes (halite),
or into rhombs (calcite and dolomite).
Other minerals have different types of cleavage.
For more mineral photographs and more information on cleavage see
- Fracture - irregular breakage not related to planes of weakness in the mineral. Some minerals, such as
quartz and olivine, do not have cleavage.
They have a type of fracture called conchoidal fracture.
Conchoidal fracture produces curved breakage surfaces, such as would be seen
on arrowheads or chipped glass.
- Magnetism - A few minerals are magnetic.
They are attracted to a magnet, or they act as a natural magnet,
attracting small steel objects such as paperclips.
Reaction to acid - The carbonate minerals react with diluted
hydrochloric acid (HCl) by effervescing or fizzing, producing bubbles of
carbon dioxide gas.
Examples: Calcite fizzes readily in hydrochloric acid.
Dolomite will fizz if it is first scratched and powdered.
- Taste - Some minerals have a distinctive taste.
Example: Halite has a salty taste. It is used as table salt.
- Flexibility - Some minerals can be bent.
Examples: Muscovite and biotite mica are elastic. When bent they return to their original shape.
Gypsum is flexible. It bends and stays bent.
- Feel - Some minerals have a distinctive feel to the fingers.
Example: Talc has a soapy feel.
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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA
September 9, 2005