Chapter 9 Flashcards
Chain Silicate Structure
Forms when 2 of a tetrahedron’s oxygen atoms are shared with adjacent tetrahedra to form a chain structure.
Each chain has a net excess if negative charges.
Silicon to Oxygen Ratio is 1:3 (pyroxene)
Isolated silicate structure
Silicate minerals that are structured so that none of the oxygen atoms are shared by tetrahedra.
Individual silicon-oxygen tetrahedra are bonded together by positive charged ions.
(Olivine) contains 2 ions of either Mg2+ or Fe2+ for each silicon-oxygen tetrahedron.
Sheet Silicates
Structure in which each tetrahedron shares 3 oxygen atoms to form a sheet.
(Mica and clay) positive ions hold sheet together are sandwiched between silicate sheets.
Framework Silicates
Structure when all four oxygen atoms are shared by adjacent tetrahedra.
(Quartz and Feldspars).
Cleavage
Is the ability of a break, when struck or split, along preferred planar directions.
A mineral tends to break along certain planes because the bonding is weaker in those regions.
Quality of cleavage can vary also, adds to diagnostic value. Perfect cleavage means cleaner break.
Quartz has no cleavage because all bonds are equally strong in all directions.
Most useful diagnostic tool because it remains identical from a given mineral sample to another.
Silicon-Oxygen Tetrahedron
SiO4(4-)
Basic building block of a crystal.
Atoms are strongly bonded together and the negative charges exceed the positive charges.
Polymorphs
Different crystal structures that have the same composition.
Ex: graphite and diamond.
Ferromagnesian Minerals
Composed of iron/magnesium bearings.
Often black and dark green in color.
Ex: Augite, olivine, biotite…
Physical Properties of Minerals
Color Streak Luster Hardness External Crystal Form Cleavage Fracture Specific Gravity Striations
Luster
The quality and intensity of light that is reflected from the surface of a mineral.
Either Metallic or Non-metallic
(aka. vitreous/ glassy) or earthy lusters possible.
Luster is described by comparing to familiar substances.
Hardness
The “scratchability” of a mineral.
Can be tested fairly reliably, by seeing if the harder substance can create a groove or scratch on a softer substance.
Ex: Quartz can always scratch calcite or feldspar, so it is harder on the hardness scale.
Mohs’ Hardness Scale
Measures of force necessary to make an indentation on a substance.
10 minerals are designated as standards of hardness::
Talc: softest on scale Gypsum: Calcite: fingernail hardness Fluorite: copper coin hardness Apatite: knife blade/ window glass Feldspar: Quartz: Almost steel file hardness Topaz: Corundum: Diamond: hardest on scale
External Crystal Form
Is the set of faces that have definite geometric relationship to one another.
Crystals commonly consist of several types of forms combined to generate the full body of each sample.
Regularity in nature can be good diagnostic property, as minerals that display crystal form have sets of angles for adjacent faces that never vary from sample to sample for a given mineral.
Fracture
The way a substance breaks where not controlled by cleavage.
Minerals that have no cleavage(quartz and garnet) often have an irregular fracture.
Minerals with cleavage can also “fracture” along directions other that of the cleavage.
Conchoidal Fractures: breaks along curved fracture surfaces (looks like clam shell).
Specific Gravity
Common expression of Density.
Ratio of a mass of a substance to the mass of an equal volume of water.
Liquid water: 1 Ice: 0.9 Quartz: 2.65 Feldspars: 2.56 - 2.76 Gold: 19.3
Striations
Straight, parallel lines on the flat surfaces of one of the two cleavage directions.
Ex: plagioclase, tourmaline.
Acid Fizz Test
One chemical reaction that is routinely used for identifying minerals…
Adding a drop of weak acid(those containing CO3 anion) to a mineral sampling and looking for fizzing(CO2 gas).
How do various feldspars differ from one another chemically?
Depends mainly on size/atomic radii of elements. Lined up based on what ions can sub into for others.
Alkali feldspars vs Plagioclase feldspars differ chemically by having K+, Na+, or Ca+ replace each other at the same location in the crystalline structure.
What is the distinction between cleavage and external crystal form?
Both produce flat sides and sharp edges, but cleavage is how the mineral breaks, whereas crystal form is how the mineral grows.
Cleavage faces are usually much shinier and flatter than crystal growth faces.
Distinguish following on basis of physical properties:
Quartz/feldspar
Calcite/feldspar
Muscovite/feldspar
Pyroxene/feldspar
Feldspars: cleavages at 90 degree angles, doesn’t fizz during acid test, white or pinkish in color.
Quartz: usually has conchoidal fractures
Muscovite: 1 cleavage, thin flexible sheets.
Pyroxene: dark green in color
Calcite: fizzes with acid test.
What major factor controls chemical activity between atoms?
Temperature. Also, valence electrons.
What are the three most common elements in the Earth’s crust?
Oxygen 47%
Silicon 28%
Aluminum 8%
