Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Minerals
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 possible.
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.
Metallic Luster
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.
High Density
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). 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.
Hardness
If a mineral can scratch glass and is non-metallic, suspect a silicate first, then perhaps one of the 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.
Cleavage
- 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
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 yellow-green.
- 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
Geologic Setting
Sedimentary rocks
- Common minerals: calcite, dolomite, gypsum, glauconite, celestite, barite, fluorite
- Arid or well-protected settings: halides and sulfates, also be alert for borates, nitrates in extremely dry settings
Igneous rocks
- 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.
- 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
Metamorphic Rocks
- 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, anthophyllite, cummingtonite
- High temperatures: staurolite, garnet, kyanite, sillimanite, wollastonite, forsterite, periclase, graphite.
- High pressures: glaucophane, jadeite, lawsonite
Rocks
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 characteristics:
Sedimentary
- Has obvious stratification
- Very soft (easily scratched by a knife)
- Obviously made of particles cemented together
- Contains fossils
Igneous-Volcanic
- Contains numerous bubble-like cavities that may or may not be lined with minerals
- Has obvious bubbly or frothy texture
- Is fine-grained, uniform in texture, and hard
- Glassy or highly vesicular rocks are almost always igneous
Igneous-Plutonic
- Made of discrete mineral grains locked together (may be loosened by weathering)
- Contains large crystals in a finer-grained mass
- Rock mass obviously cuts across other rock structures.
Metamorphic
- 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 sedimentary features
- Contains distinctive metamorphic minerals like garnet or kyanite
- May often have features of original rock but is recrystallized or chemically changed.
Sedimentary Rocks
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 minerals.
- 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
Sedimentary-Volcanic
These are rocks created by volcanic action but deposited by mechanisms similar to sedimentary rocks. Some people classify them as volcanic, others as sedimentary.
- 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 lahars.
Igneous-Volcanic
- Dark, fine-grained massive rock: basalt or andesite. Difficult to tell apart without chemical or microscopic study. Olivine is most likely to indicate basalt.
- 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
Igneous-Plutonic
- 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
Metamorphic
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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