Identifying Rocks and Minerals

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. Weathered biotite is yellow and sometimes mistaken for gold. Any platy "metallic" mineral is most likely mica.

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). 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.

Hardness

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.

Cleavage

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.

Geologic Setting

Sedimentary rocks

Igneous rocks

Metamorphic Rocks


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

Igneous-Volcanic

Igneous-Plutonic

Metamorphic


Sedimentary Rocks

Obvious Fragments Visible

No Fragments Visible

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.


Igneous-Volcanic

Igneous-Plutonic


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.


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Created 26 September 2001, Last Update 01 September 2011

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