Felsic






Silicon to Oxygen ratio: (1:2)
Name comes from “feldspars”
Felsic minerals cover most of
Earth’s surface
Light colored: pink, white, yellow
Less dense
Lower melting temperature:
easy to melt, hard to crystallize
Ultramafic/Mafic






Silicon to Oxygen ratio: (1:4)
Name comes from “magnesium and
iron (Latin: ferrum)”
Make up most of mantle
Dark colored: black, green
Denser
Higher melting temperature
Quartz











Formula: SiO2
Most felsic mineral
Has lots of forms, many of which don’t look alike
Physically and chemically durable; more chemically durable than diamonds
7 on Mohs Scale of Hardness; is the hardest common mineral, as only gemstones are harder
 Gemstone- attractive minerals used to make jewelry
Heavily represented in sedimentary rocks because everything else gets broken down
99.9% of quartz is NOT in crystalline form
In the real world, contaminants, temperature changes, etc. keep crystals from forming in ideal
shape
Crystal face- flat side
Termination- point at crystal’s end
Cleavage- tendency of mineral to break along flat surfaces
 Quartz has no cleavage; instead, it has conchoidal fracture, a curved breakage with ripple
patterns that also occurs in obsidian

Quartz crystals are hexagonal (6 crystal faces)
Some varieties of quartz:
Milky- grayish
Crystal- mineral grains distinctive; transparent
Rose- pink
Citrine- orange, crystalline
Amethyst- purple
Agate- has layered bands
Jasper- darker agate
Chalcedony- rose shapes
Chert- forms on seafloor
Smoky- structural damage by radiation causes color
Tigerseye- orange with optical effect
Aventurine- green
Feldspars





Taken collectively, most common minerals in crust are feldspars
Less felsic than quartz
Not as durable as quartz with only medium chemical durability
Hardness of 6 on Mohs Scale
Feldspars at Earth’s surface decay into clay from chemical weathering over time




Orthoclase (K-spar)- potassium feldspar; cleavages intersect at 90°
Plagioclase- Sodium feldspar and Calcium feldspar
3 types of potassium feldspar: orthoclase, sanidine, microcline
Can use color to distinguish orthoclase and plagioclase 90% of the time
 Orthoclase- pink to orange


Plagioclase- white
Labradorite- pretty, iridescent plagioclase
 Can sometimes be mixed up with quartz
Apart from color, cleavage is the best way to tell them apart
Micas







Group of minerals with hardness of about 3 or 4 on Mohs Scale
Generally flat, sheeted ,flaky, and shiny minerals that can be peeled with fingernails (part of the Mica
experience)
Has one prominent cleavage
Called sheet silicates because of the sheet-like arrangement of its atoms
Often found in streambeds (ends up on top of sediments because it’s light) and in sand
Was used for windows because it’s naturally flat and regular; also used for its dielectric strength
3 common types :
 Muscovite (name after Moscow, where it was mined)- light, silvery color; little more felsic than
biotite. Formula: KAl2(AlSi3)O10(OH)2

Biotite- black to brown color; little more mafic than Muscovite.
Formula: K(Mg, Fe)3(Al, Fe)Si3O10(OH, F)2

Lepidolite- purple color; has structure and chemical content similar to those of muscovite
(sometimes same mineral grain is silver, then purple, then silver); the main ore of lithium;
an indicator of gemstones

Quartz, feldspars, and micas make up the bulk of felsic, igneous rocks (like granite)

Amphiboles/Pyroxenes- both big, complicated mineral groups.
 Most common amphibole is Hornblende
 Most common pyroxene is Augite
Hornblende




Black, shiny, lustrous
Amphibole: less dense with evident cleavage planes at 60°/120°
Hardness of 6
Felsic
Augite




Has some similar properties to hornblende: black, shiny, lustrous
Pyroxene: denser with cleavages meeting at 90°
Hardness of 6
Mafic: therefore, it occurs in different rocks from hornblende
Olivine









Most mafic mineral
(Mg, Fe)2SiO4
Green to yellow color
Translucent
Vitreous (glassy) texture
no cleavage
Weathers rapidly at earth’s surface into oxides and serpentine
Most of mantle and lower crust is olivine; found in basalt
Iron olivine is greener, magnesium olivine is yellower
Peridot- gem-quality olivine
Dunite- rock that’s 90% olivine
Miscellaneous Rock Facts
 Making rocks wet brings out color and contrast
 Polishing a rock’s surface makes this wet-effect permanent
 Cooling something gradually forms few nucleation sites, allowing a small number of crystals
to grow; cooling something fast creates several nucleation sites, leaving lots of small crystals
 That’s why liquid nitrogen ice cream takes so much creamier (smaller crystals)
 Earth’s radius is 4,000 miles (6,380 km)
 Zoning- subtle changes in chemical composition in different rock layers


Mass spectrometer- device that identifies the chemical composition of a compound or sample
based on the mass-to-charge ratio of charged particles; the ratio is calculated by passing the
particles through electric and magnetic fields
Polarizing microscope- used to identify rocks and minerals in thin sections; optical properties
of the minerals in the thin section alter the color and intensity of the light as seen by the
viewer
Olivine and feldspar seen under a polarizing microscope:
Non-silicates- minerals not made up of silicon and oxygen
Calcite






CaCO3
Almost always biogenic (formed by organisms); lots of organisms build calcite shells
Seashells and coral reefs are made of calcite
 Plankton have shells of calcite
 Coquina- made of shells stuck together
Majority of world’s carbon found in calcite (more than that in living things, air, fossil fuels,
and ocean combined)
Test for calcite: apply hydrochloric acid to mineral, and it will bubble
 CaCO3 + 2HCL  CO2 + H2O + CaCl2
Marble, limestone, chalk, and travertine are all made up of calcite
Limestone- sedimentary rock that’s mostly calcite (generally from shells); most common
sedimentary rock not made of debris; main ingredient in concrete
Marble- metamorphic product of squeezing and heating limestone (intense pressure and heat)
Chalk- made of microscopic pieces of calcite shed from micro-organisms
Travertine- calcite formed in layers, often in found hot springs
Hematite









Rust (Fe2O3)
Distinctly darkly pigmented (dark red/brown rust color)
Usually in the form of a microscopic powder and a trace component in rocks
 Gives rocks a reddish hue
Mars is red because of the hematite on its surface
Hematite is the end result of chemical weathering of iron (oftentimes found in mafic rocks)
Main ore of iron
Streak- color of the powder produced when it a mineral is dragged across an unweathered
surface
 Ceramic tiles are often used as streak plates for streak tests
 Hematite has a distinctive dark red streak, even though solid hematite is black
BIF- banded-iron formation; sedimentary rocks with structures consisting of repeated thin
layers of iron oxides, either magnetite or hematite, alternating with bands of iron-poor shale
and chert
Soil in wet places has hematite
Hematite:
Magnetite





Fe3O4 (Fe3+, Fe3+, Fe2+ with O2-)
Most common and strongest of all naturally magnetic minerals
 Magnetite grains are positioned along Earth’s magnetic fields
“lodestone”- naturally magnetized magnetite that was used as an early compass
 Made when lightning strikes magnetite; very rare)
 Can create artificial magnetic magnetite by heating it up and cooling it down quickly in a
magnetic field
Powder is black; leaves a black streak
Similar to hematite: exists as microscopic powder in other rocks
 You can distinguish between hematite and magnetite by using a streak test or a magnet
 Turns into hematite after being subject to weathering on earth’s surface
Hardness- how difficult it is to scratch something; different from strength
 Diamond is hard, but very brittle (will shatter if hit with hammer)
Mohs Hardness Scale
10- Diamond
9- Corundum
8- Topaz
7- Quartz
6- Orthoclase feldspar
5- Apatite
4- Fluorite
3- Calcite
2- Gypsum
1- Talc
Mnemonic: The Gay Cowboy Found An Old Quaker To Corundum Diamond
Talc






Softest sort of common mineral
Whitish, greenish, grayish color
Is an amphibole
Slippery texture
Used in talcum (baby) powder
Can carve it up with fingernails (hardness of 2.5)
Gypsum






Calcium sulfate: CaSO4 or CaSO4 • 5H2O (hydrate- holes in crystal structure that can trap
water)
Is an evaporite- forms when water evaporates (like in a desert’s dry lake beds)
Looks like calcite
Very soft: can be scratched with fingernails; usually powdery
Sometimes crystallizes, as in selenite
Used to make plaster, like in drywall
Gypsum:
Selenite:
Diamond







Just carbon (C), like charcoal
Can only form naturally 10 km below Earth’s surface; probably not rare in the mantle
Not stable at earth’s surface: slowly turning into graphite
Only found in “kimberlites”- weird blocks of rock raised quickly from the mantle
Very shiny: has highest known index of refraction in a naturally occurring mineral
 Index influences how much light bends in mineral
 Also influences the critical angle, angle of incidence, above which all light is reflected
and bounces
We have the means to make large synthetic, gem-quality diamonds at cheap prices, but the
De Beers Group is against it
Industrial diamonds are produced for stuff like heat sinks
Corundum



2nd hardest naturally occurring mineral
Al2O3
Generic name that includes the colorful, transparent gemstones: ruby and sapphire
 Rubies- red (colored by chromium)


Sapphires- color other than red; can be cooked to become blue/bluer
Synthetic corundum used to make watch faces (scratch-proof) and knives
 Anything with hardness 8 or above is pretty much scratch-proof since it cannot be
scratched by sand (quartz)