by Harold L. Levin
Chapter 4 - page 2
Rocks and Minerals: Documents that Record Earth's History
There are more than 3,000 minerals on the Earth, but comparatively few are common
and make up most of the rocks. We will discuss the common rock-forming minerals.
They can be divided into two groups, the silicates and the non-silicates.
Earth's crust is dominated by 2 chemical elements:
- Oxygen (46.6% by weight)
- Silicon (27.7% by weight)
These elements help make up the dominant group of rock-forming minerals,
the silicate minerals.
The silicate minerals are based on a crystal structure
that involves four oxygen atoms arranged in pyramid-like shape, surrounding a smaller silicon atom.
This structure is called the silicate tetrahedron.
Minerals that do not contain silicon are called non-silicates.
- Feldspar group - Dominant mineral in Earth's crust. 60% by weight.
Feldspars have two directions of cleavage at 90o, with flat, glassy rectangular surfaces.
Color ranges from white to pink to gray. May also be green.
Common in igneous rocks such as granite and basalt.
The feldspars are group of minerals.
Two major types:
- Orthoclase feldspar (or potassium feldspar) group - KAlSi3O8
Orthoclase feldspar (or potassium feldspar). Photo courtesy of Pamela Gore.
- Plagioclase feldspar group - A solid-solution series with a range of compositions in between a
calcium-rich end member and a sodium-rich end member. Striations (thin parallel grooves) are present on
cleavage surfaces in plagioclase feldspars.
- Calcium-rich end member = anorthite (CaAl2Si2O8)
- Sodium-rich end member = albite (NaAlSi3O8)
Quartz (SiO2) - Second-most abundant mineral in Earth's crust.
A hard mineral (scratches glass) with glassy luster and
conchoidal fracture. Color is variable.
Colorless, white (milky quartz), gray to brown (smoky quartz), pink (rose quartz),
or purple (amethyst). May also be blue or green.
Forms six-sided, elongated crystals.
Common in granite, and in many sands (particulary in humid eastern US),
because of its durability in the weathering environment.
Major constituent of quartz sandstone and quartzite.
Chert is a sedimentary rock composed of microcrystalline quartz.
Quartz crystals. Photo courtesy of Pamela Gore.
Mica group - Perfect cleavage in one direction causing it to split into thin sheets.
Common in metamorphic rocks, particularly schists.
- Muscovite - Colorless or silvery-colored mica.
- Biotite - Black or dark brown mica (contains Mg and Fe).
Muscovite. Photo courtesy of Pamela Gore.
Amphibole group - Two directions of cleavage, not at 90o, producing narrow, elongated crystals.
Typically dark in color (black or dark green).
Example: Hornblende. Contains Mg and Fe.
Pyroxene group - Two directions of cleavage at 90o. Typically dark in color.
Example: Augite. Contains Mg and Fe.
Pyroxene. Photo courtesy of Pamela Gore.
Olivine - Olive green color and glassy texture. No cleavage. Conchoidal fracture.
Contains Mg and Fe.
Olivine is the main constituent of the ultramafic rock, peridotite (birthstone = peridot).
Olivine in peridotite. Photo courtesy of Pamela Gore.
Clay minerals - A group of minerals formed through the weathering of feldspars
and some other minerals.
Typically very fine-grained flakes, dull, earthy luster, soft, smooth feel.
Example: Kaolinite, a white clay with many economic uses.
Kaolin, a white claystone composed of kaolinite. Photo courtesy of Pamela Gore.
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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA
September 9, 2005