Identifying Rocks and Minerals
Steven Dutch, Natural and Applied Sciences, University
of Wisconsin - Green Bay
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Before you identify rocks, you have to be able to identify the minerals that
make them up. Here's a strategy to follow. These are guidelines designed to get
you to the most likely identifications fastest. Bear in mind that exceptions are
First Principle: Suspect the most likely mineral first. There's a
saying among medical students that applies here, too: "When you see hoof
prints, think horses, not zebras." Doctors could waste a huge amount of
time and money if they tested for every rare disease that might produce a given
set of symptoms, but almost always the more common explanation is correct. The
same is true of identifying rocks and minerals.
Suspect a sulfide first, especially if you can detect a sulfur smell. Next
most likely, an oxide, then perhaps a metallic element or compound of one of the
semi-metals (As, Se, Bi, Te). Non-metallic luster could indicate any other
group. However, some of the sulfide minerals are non-metallic, notably
sphalerite, orpiment, realgar and cinnabar. The last three are identified by
their bright colors.
Not many minerals can appear either metallic or non-metallic. Hematite and
sphalerite are the most common. Muscovite mica can appear silvery. Weathered
biotite is yellow and sometimes mistaken for gold. Any platy "metallic" mineral
is most likely mica.
Suspect a sulfide first, especially if you can detect a sulfur smell. Next
most likely, an oxide, then perhaps an element or compound of one of the
semi-metals (As, Se, Bi, Te). Note that this property pretty much goes along
with metallic luster. Few non-metallic minerals have high density;
barite and sphalerite are the most common. Light colored dense minerals are most
likely barium or lead minerals, or one of the non-metallic sulfides.
If a mineral can scratch glass and is non-metallic, suspect a silicate first,
then perhaps one of the hard oxide minerals like corundum or rutile. Always suspect
quartz first, then a feldspar. If it's metallic in luster, suspect an oxide.
Very soft non-metallic minerals that can be scratched easily with a knife are
most likely to be carbonates, halides or sulfates.
- Cubic or octahedral, soft, non-metallic: suspect a halide
- Cubic and metallic: suspect a sulfide
- Rhombohedral: suspect a carbonate. Check with acid test
- Other soft mineral with good cleavage: suspect a sulfate
- Thin sheets: suspect a mica
- Hard splintery mineral: suspect a chain silicate
- Hard, with good blocky cleavage: suspect a feldspar
Color is far down the list because it is easily the least reliable
characteristic of minerals. Color can always be due to an impurity or surface
stain. As an undergraduate, I was once asked to try to identify a hard bright
blue mineral. I even had X-ray data to help. After running through all the
copper minerals with no luck, I looked at the X-ray data for all minerals and
found a perfect match with diopside. We had a common pyroxene mineral that is
normally white, but in this case was stained by copper. So always suspect that
color may be due to impurities.
- Bright blue to green: suspect a copper mineral. Dull greens are usually
not copper greens, nor are blues that have a violet cast.
- Earth tones are almost always due to iron, either as a principal
ingredient or as an impurity or surface coating.
- Bright yellow, orange or red: suspect one of the non-metallic sulfides,
then one of the transition metal radicals (chromate, vanadate, etc.). A few
oxides are also brightly colored. Some uranium minerals are bright yellow or
- Pink: if hard, suspect potassium feldspar. The common manganese minerals
rhodonite (silicate) and rhodochrosite (carbonate) are also pink, and
manganese can stain other minerals pink as well. Some lithium silicates are
also pink or lavender.
- Black or dark green: if hard, suspect a ferromagnesian silicate
- Pea green, especially in granite or a metamorphic rock: epidote. Almost
- Dark green mineral in sandstone: glauconite
- Light blue mineral in carbonate rocks: celestite
- Light blue mineral with long crystals in metamorphic rocks: kyanite
- Common minerals: calcite, dolomite, gypsum, glauconite, celestite, barite,
- Arid or well-protected settings: halides and sulfates, also be alert for borates, nitrates in
extremely dry settings
- Quartz never occurs with olivine, nepheline, leucite, melilite or corundum
- Common minerals: Quartz, feldspars, pyroxenes, amphiboles, olivine, micas.
- Granitic rocks may include epidote, tourmaline, beryl, apatite, topaz,
zircon, and sphene
- A pea-green mineral in granitic rocks is almost always epidote.
- An intensely black mineral, especially with elongated crystals, is
most likely tourmaline. A rounded triangular cross-section clinches it.
Tourmaline is far deeper black than biotite, pyroxene, or amphibole.
- In pegmatites, you may find lithium and rare-earth minerals.
- Mafic rocks may contain calcic plagioclase, pyroxenes, amphiboles,
olivine, nepheline, leucite, corundum, magnetite, spinel, or zeolites
- May include any minerals found in the parent rocks
- Distinctive metamorphic minerals include:
- Low temperature: chlorite, talc, chloritoid
- Low pressures: andalusite, cordierite
- Medium temperatures: actinolite, tremolite, epidote, diopside,
- High temperatures: staurolite, garnet, kyanite, sillimanite,
wollastonite, forsterite, periclase, graphite.
- High pressures: glaucophane, jadeite, lawsonite
Identifying rocks is less critical in some ways than identifying minerals. A
dense, gray mineral is either galena or it isn't. On the other hand, sandstone
can grade into siltstone, limestone into dolostone, gabbro into diorite. If a
rock is on the borderline between two types, it's usually not all that critical
where you place it.
The Three Great Rock Families
Suspect a rock is of a given type if it has one or more of these
- Has obvious stratification
- Very soft (easily scratched by a knife)
- Obviously made of particles cemented together
- Contains fossils
- Contains numerous bubble-like cavities that may or may not be lined with
- Has obvious bubbly or frothy texture
- Is fine-grained, uniform in texture, and hard
- Glassy or highly vesicular rocks are almost always igneous
- Made of discrete mineral grains locked together (may be loosened by
- Contains large crystals in a finer-grained mass
- Rock mass obviously cuts across other rock structures.
- Has a fine texture with an obvious directional grain (foliation)
- Has obvious bands, streaks or clumps of different minerals
- Is made mostly of quartz or calcite but is coarse-grained and lacks
- Contains distinctive metamorphic minerals like garnet or kyanite
- May often have features of original rock but is recrystallized or
Obvious Fragments Visible
- Borderline of visibility: siltstone
- Sand-sized: sandstone (did you really need me to tell you that?)
- Abundant feldspar: arkose
- Rock fragments and mafic silicates: graywacke
- Pebbles: conglomerate
- Angular pebbles: breccia
No Fragments Visible
- Soft and fizzes vigorously in acid: limestone
- Soft, often buff color, fizzes slowly in acid: dolostone
- Very soft, light color, may be granular with obvious cleavage visible:
gypsum or rock salt. Confirm salt with taste test. Consider other evaporites like
borates in arid areas.
- Mudrocks are opaque, usually very soft, but may be quite hard if
well-cemented. They may fizz if they have significant carbonate
- Vigorous fizzing in acid: marl
- Obvious sheet-like or flaky bedding: shale
- Massive but made largely of clay: argillite
- Gritty texture to feel or when bitten (seriously!), but particles
borderline in visibility: siltstone
These are rocks created by volcanic action but deposited by mechanisms
similar to sedimentary rocks. Some people classify them as volcanic, others as
- Unconsolidated powder with occasional larger fragments: ash
- Small rock pellets: lapilli
- Large fragments, hand-sized and up: bombs
- Consolidated ash or lapilli: tuff
- Large broken fragments in finer material; may be ejecta blocks, mudflow,
or due to explosion: volcanic breccia. Mudflow deposits are often called
- Dark, fine-grained massive rock: basalt or andesite. Difficult to tell
apart without chemical or microscopic study. Olivine is most likely to
- Fine-grained massive rock, may be any color. Often porcelain-like in
texture and may have quartz or feldspar visible: rhyolite.
- Glassy: obsidian. If it is the cooling crust of a lava flow call it by the
volcanic rock name, for example, basaltic glass.
- Frothy, but still sinks readily: scoria
- Frothy: floats or is only a bit heavier than water: pumice
- Contains quartz and potassium feldspar: granite
- Extremely large crystals: pegmatite
- Fine, sugary dike or sill rock: aplite
- Plagioclase more abundant than potassium feldspar: granodiorite
- Contains abundant potassium feldspar but no quartz: syenite
- Dominated by plagioclase feldspar: diorite or gabbro
If the original rock type can be recognized, the rock can be described by
prefixing meta- to it: metaconglomerate, metarhyolite, metabasalt, etc. Often
this is the only way of naming the rock.
- Fine platy texture but no visible grains: slate
- Fine platy texture with a visible sheen: phyllite
- Coarse platy texture, visible mica and other minerals: schist
- Obvious bands and streaks of minerals: gneiss
- Dark greenish or bluish, massive or weakly foliated: greenstone
- Made mostly of amphibole, usually aligned: amphibolite
- Massive and made mostly of quartz: quartzite
- Massive and made mostly of calcite or dolomite: marble
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Created 26 September 2001, Last Update
01 September 2011
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