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Rock Identification Cheat Sheet – works no more than 75% of the time
Rocks are way too complex for this type of treatment. There are thousands of minerals. This is
the most basic guide possible but it will get you through most rocks you will find. All rocks start
out as igneous or sedimentary rocks - any and everything that can happen to those will make a
metamorphic rock - so that category is the most diverse and difficult to identify. Correctly naming
a rock depends on being able to identify its constituent minerals and textures, so the finer-grained,
the more difficult. When in doubt, describe the minerals and textures as far as you can and go to
somebody else to help you get the rock name.
For any rock description, include:
• Minerals present, in order of abundance (crystals or grains)
• Determine whether Sedimentary (grains are thrown together w/ pores between) or
Crystalline (crystals have grown together and boundaries are interlocking) – remember there is a
transition here when dealing with lower grades of metamorphism
• Fabrics - determine whether grains/crystals are aligned in planes (foliation or bedding).
black–brown–green, stubby
10. sulfides and oxides - dark metallic things
which are common trace minerals - usually
don’t give much info about the rock type
Basic super common minerals comprising
∼100% of the crust of the earth
1. quartz – hexagonal, no cleavage, harder
than steel, usually clear or white to light
gray
2. plagioclase feldspar – white or gray, may
have planar or longitudinal twinning,
euhedral shape = rectangle
3. alkali feldspar – pink or white, may have
“plaid” or wavy laminae
4. muscovite – colorless mica, looks silvery
5. biotite – black or brown mica, shiny
6. chlorite – dark green mica (low-grade so
often fine grained)
7. carbonates – (calcite or dolomite)
rhombehedral (60-120◦ cleavage), soft.
Very fine grained (sedimentary) to very
coarse (metamorphic).
8. “hornblende” – any dark green to black
amphibole, usually long crystals with
diamond shaped cross section.
9. pyroxene – 90◦ cleavage,
Minerals which give away the rock type
1. glaucophane – blue to violet amphibole:
it’s a blueschist
2. talc – white/green, sparkly mica and very
very soft/slippery –mafic to ultramafic
rock
3. serpentine – esp. antigorite (green to
black, greasy luster, very soft) –
ultramafic igneous rock
4. glauconite – green to black grains - made
from poo of marine worms – sedimentary
rock
5. garnet - red to purple to brown, round
and faceted, metamorphic rock (schist or
gniess)
6. olivine – bright to very dark green,
associations - mafic igneous rock
Basic groups of rocks to know
A. Igneous rocks Crystals are interlocked, little or no porosity. Some crystals have their natural
“euhedral” growth shapes, especially the very large crystals. Usually crystals are randomly
oriented (no alignment or fabrics). Roughly divided by two criteria.
• Grain size. Plutonic: solidified slowly–coarse grains; and volcanic: solidified quickly–most
of rock is too fine to see crystals. Porphyritic: big grains in finer matrix (bimodal grain size)
• Average color. Felsic: Has quartz and either or both of the feldspars, dominated by these,
so usually light colored (pink, white or light gray). Dark minerals restricted to biotite and
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hornblende. Mafic: No quartz, no alkali feldspar. Dark minerals dominate and may include
pyroxenes, olivine. Color is dark gray, black or blackish-greenish.
• Mafic and Plutonic = peridotite, gabbro
• Mafic and volcanic = basalt
• Felsic and Plutonic = granite (granodiorite, monzonite, tonalite)
• Felsic and volcanic = rhyolite, dacite
B. Sedimentary rocks are composed of some combination of pebbles, cobbles, sand, silt, clay,
and carbonate. Pebbles and cobbles are fragments of any kind of rock. As the grains break down
to smaller sizes, the rock bits get broken down to individual mineral fragments. So - sand is
usually single crystals of quartz or feldspar, or very fine grained rocks (lithic fragments). Silt is
quartz or feldspar. Clay grains are much smaller and have different physical properties - e.g. mud.
The rocks are named according to the size and composition of their component grains.
• If it’s mostly calcite/dolomite, call it a limestone/dolostone. done.
• If it’s got a lot of grains over ∼5mm, call it a conglomerate. if they’re very angular, a
sedimentary breccia.
• Sandstones (grains ∼0.5-2mm) are named by the dominant grain composition and whether
there is a muddy matrix. Arenite for no matrix. Graywacke for 15% clay or above. Sand grains
divided into quartz, feldspar, and lithic (e.g. other). So: quartz arenite, lithic wacke, etc.
• If you can’t see the individual grains, it’s a siltstone, mudstone (mixed silt and clay), or
shale (claystone). Oral textural analysis: Nibble on the rock a bit. If it’s gritty, you’ve got silt. If
it’s smooth on your teeth, you’ve got clay. If it breaks your teeth and cuts your lip, it’s chert.
C. Metamorphic rocks are any of the above that have been cooked and smooshed. You can
make up your own name for them using the formula name = dominant mineral(s)+fabric. If
you’re clever, you can name the same rock “meta-X” by identifying it’s protolith. Sometimes you
can see the original textures of a rock even after new metamorphic minerals have grown, this can
help identify the protolith.
• Rocks which are mostly platey-shaped mica (muscovite, biotite, clorite) will have a flakey
foliation = schists. If muscovite dominant, the rock is a meta-shale. If chlorite or talc dominant,
the rock is a meta-basalt or gabbro and you can call it greenschist.
• If the rock is mostly hornblende, it’s an amphibolite, and its protolith is mafic.
• Micas have water, unstable at higher temp so when the same rocks are baked hotter, they
lose their platey minerals and their flakey foliation. The Fe and Mg from the micas will form new
dry (anhydrous) crystals of hornblende, garnet or pyroxene.
• Quartz and feldspar are stable at all P/T so felsic igneous rocks and sandstones tend to
make really boring metamorphic rocks which look more like the original, only squished:
orthogneiss for meta-igneous and paragneiss for metasedimentary.
• Some minerals, like garnets, like to grow really big compared to the surrounding matrix –
texture: porphyroblastic
• If all the grains are the same size and rock feels sugary, it’s been annealed at high
temperature and it’s a granulite.
A few more generalizations – even less reliable than the ones above
• Micas and amphiboles – if they are present but not aligned, and smaller than the
quartz/feldspar grains, it’s probably a plutonic igneous rock. if they are abundant and well
aligned, the rock is a schist. If they are minor component and well-aligned, the rock is a gneiss
• If the minerals are well aligned, it’s probably metamorphic.
WARNING: All bets are off when rocks are weathered 1) by the weather 2) by fault
action/heavy shearing 3) by hydrothermal fluids
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