Review:

Minerals are:

• naturally occurring
• inorganic
• a solid
• have an orderly internal structure

Arrangement of Ions in Table Salt (Halite)

• have a definite chemical composition

• and have unique properties that make them different from the elements that comprise them

• rocks - a mixture of minerals, with each mineral retaining its own identity

(1) Crystal Form

• crystal form is the external expression of a mineral's orderly internal atomic arrangement
• if the mineral has sufficient space while forming, it will develop well formed crystals (e.g., pyrite w/cubic faces; quartz w/hexagonal faces)
• if space is limited, crystal growth is stunted and the mineral will not exhibit its crystal form

(2) Color

• an obvious but unreliable property of a mineral
• a mineral which comes in several colors, has "exotic coloration" (e.g., quartz)

• clevage is the tendancy of a mineral to break along certain planes where the bonds are weak (e.g., the micas)
• some minerals have several clevage planes, some have poor clevage, some have no clevage (e.g., quartz)

Sheet-type Clevage

• minerals without clevage exhibit fracture
• one such specific type of fracture is "conchoidal fracture" - broken glass like
• other fracture types: splintering, fibrous, irregular

• the ratio of the minerals density to that of water (the density of water = 1 g/cm³)
• specific gravity = density of mineral / density of water
• the specific gravity of common rocks = 2.5 to 3.0

• specific gravity of galena = 7.5; specific gravity of  gold = 20

• streak is the color of a mineral in its powdered form
• it is obtained by rubbing a mineral across a porcelain "streak plate"
• while mineral color may vary form sample to sample, streak typically does not

• luster is the way in which a mineral reflects light
• the capacity to reflect light depends on how smooth a crystal surface is at the atomic level
• e.g.: metals have metallic luster; nonmetals: glassy (vitreous), pearly, silky, earthy (dull)

• a measure of the resistance of a mineral to scratching
• a mineral's hardness can be compared to that of minerals having a known hardness - "Mohs Scale"

Mohs Scale of Mineral Hardness

B. Mineral Groups

Recall: only eight elements comprise the bulk of the rock-forming minerals, and represent over 98% (by weight) of the continental crust

• the two most abundant elements-silicon & oxygen-comprise nearly 3/4 of the Earth's continental crust

• silicon & oxygen combine to form the framework of the most common mineral group: the silicates

• most rocks are made of silicate minerals!! I.O.W., they are the most common mineral group!!

• silicate minerals: olivine, pyroxene, amphibole, mica, feldspar and quartz

• the fundamental building block of the silicates is the:

• 4 oxygen atoms & 1 silicon atom

(SiO₄)^4-
• its an ion, not a compound: its "incomplete", thus unstable

• each oxygen ion provides a charge of -2

• the silicon ion provides a charge of +4

Silicon-Oxygen Tetrahedron

• the tetrahedra become stable when positive ions are added to them (iron, magnesium, potassium, sodium, and calcium)

• these ions also connect the silicate structures (i.e., the chains, sheets, and 3-D structures)
• the feldspars are the most abundant silicate group - comprising over 50% of the Earth's crust

• quartz (SiO₂)is the second most abundant mineral in the Earth's crust

Common Silicate Minerals

(a) Crystallization Environment

• silicate minerals crystallize as the magma cools
• near Earth's surface = low temp & press.
• at depth = high temp & press.
• environment during crystallization & the chemical composition of the magma determine which minerals are generated

olivine = high temps

• thus, mineral structure and composition is indicative of the conditions that existed when the mineral was forming

• tells geologists about the environment

• have important economic value

• carbonates CO₃^2- + pos. ions - e.g., calcite (limestone), dolomite (dolostone)

• halite - "table salt"
• gypsum - used to make plaster