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I. Minerals
Definitions – Earth MaterialsA. Rock: naturally occurring aggregate of
one or more mineralsB. Mineral: naturally occurring solid with a
definite chemical composition and an orderly internal structure.
Natural: synthetic gems and other substances created by chemists are not minerals
SolidDefinite chemical composition: consists
of one or more chemical elements in specific proportions
Orderly internal structure: elements are arranged in a systematic way that is the same in every sample of that particular mineral
Minerals
Rocks and Minerals
At one time or another, you have probably held a rock in your hand
Recall the color.
Recall the size.
The word rock refers to solid mineral deposits.
Rocks are minerals, but not all minerals are rocks. Minerals are all substances that can't be classified as "animal" or "vegetable."
Minerals
Metals ores found in the earth, like gold and silver, are minerals.
Crystalline substances, like salt and quartz, are minerals.
Homogeneous natural substances, like water and gas, are also minerals.
These non-solid minerals?"
Rock: an aggregate of one or more minerals
Minerals
(a) atomic structure and chemical reactions: a review
(b) What is a mineral?
(c) Classification of minerals
(d) Silicate minerals
What is a mineral?
First we need to understand atomic structure of elements.
Atomic structure and chemical reactions
STRUCTURE OF ATOMS: Atom is the smallest unit of an element that retains the physical and chemical properties of that element.
Atom: the smallest particles of an element that retain all of the element’s chemical properties
Chemical Compounds: specific combinations of one or more elements.Atoms bond together to form compounds
a. SiO2, NaCl, PbS, Fe2SiO4, CaAl2Si2O8
All of the above are chemical formulas of minerals
b. Minerals are essentially chemical compounds; elements, in certain proportions, bonded together
A. Relative abundances of elements
Of the 92 naturally occurring elements, 8 make up >98% of the Earth’s continental crust
Most minerals in the crust are O and Si based rocks (silicates)
Most minerals in the mantle are O-Si-Fe-Mg based rocks
Composition of Earth Crust Entire Earth
1. Oxygen (O) 45.2% 29.3% (2)2. Silicon (Si) 27.2% 14.7% (3)3. Aluminum (Al) 8.0 1.2% (8)4. Iron (Fe) 5.8% 34.8% (1)5. Calcium (Ca) 5.1% 1.4% (7)6. Magnesium (Mg) 2.8% 11.3% (4)7. Sodium (Na) 2.3%8. Potassium (K) 1.7% Sulfur (S) 3.3% (5) Nickel (Ni) 2.4% (6)
Total 98.1% 98.4%
CHEMICAL REACTIONS and TYPES OF CHEMICAL BONDS
Ions = atoms that gained and lost electrons:
Ions -- after gain or loss of an electron, atoms are no longer electrically neutral.
When Na loses an electron it becomes a Na ion.
Positive ions = cations,
Negative ions = anions.
CHEMICAL REACTIONS
TYPES OF CHEMICAL BONDS
Relative sizes and charges of these elements’ atoms and ions
1. Given two elements of equal abundance, the element that will contribute more readily in a mineral is the one that “fits” better with the other elements.
2. Ions pack together as closely as possible (large negative ions with small positive ions in the “holes”.
3. If amount of one element runs low, it may be replaced by another.
TYPES OF CHEMICAL BONDS
One type of atomic bond is:
Ionic substitution: ions of similar size and shape (charge) replace one another within a crystal structure
For example Mg and Fe2+ in olivine: (Fe, Mg)2SiO4 )
Na (+1) and Cl (-1), + and – attract, so Na loses an electron and Cl gains an electron
Ionic substitution
Ions of similar size and shape (charge) replace one another within a crystal structure
Ca and Mg can substitute each other
O and Ca can not replace another
The temperature and pressure at the time of formation (Structural Variation)
Polymorphs: minerals with the same composition, but different structures because they formed under different conditions of heat and pressure
For example, diamond and graphite are both pure carbon, one very hard & one soft.
Diamond needs high pressure (>90 miles)Graphite more stable at Earth’s surface
III. Identifying Minerals
Identifying minerals is the key to geology.
We ID the minerals to know what rock it is and where it came from.
The minerals tell us a story about where the rock formed and under what conditions
A. Color: first thing you notice, but least reliable, can vary greatly (quartz, calcite, salt)Color depends on the composition and structure.
Corundum = clear Chromium = red rubies Titanium & Iron = blue sapphiresLithium = pinkCopper = green
III. Identifying Minerals
B. Luster; how its surface reflects light-- its shininess
MetallicSilkyVitreous (glassy)Earthy (dull)
C. Streak—the color of a mineral in its powered form.
Scrape the mineral across an unglazed porcelain tile (streak plate)
Different color powder than how the mineral looks alone
III. Identifying Minerals
D. Hardness: how hard it is - resistance to scratching or abrasion
Scale from 1-10
Diamond 10Steel 6.5Glass 5-6Copper penny 3.5Fingernail 2.5Graphite 1-2
E. Smell and TasteSulfur- rotten eggsHalite- salty Sylvite- bitter
F. EffervescenceRocks containing CO3 react with HCl to
form CO2 gas
III. Identifying Minerals
F. How they break
1. Cleavage: tendency to break consistently along distinct planes in the crystal structure due to weak bonds---break along flat surfaces called cleavage planes1. cubic—halite 2. sheets—micas 3. rhombus—calcite
2. Fracture—breaks are jagged and irregular
conchoidal fracture: breaks like glass -curved, shell shaped surface
Int-A.02b
Dr. Kent Ratajeski, Department of Geology and Geophysics, University of Wisconsin,
Madison
Analytical techniques
X-Ray Diffraction
Nobel Prize for determining the structure of halite (salt)
Microprobe
Ion Probe