Identifying Rocks and Minerals

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay


Minerals

Before you identify rocks, you have to be able to identify the minerals thatmake them up. Here's a strategy to follow. These are guidelines designed to getyou to the most likely identifications fastest. Bear in mind that exceptions arepossible.

First Principle: Suspect the most likely mineral first. There's asaying among medical students that applies here, too: "When you see hoofprints, think horses, not zebras." Doctors could waste a huge amount oftime and money if they tested for every rare disease that might produce a givenset of symptoms, but almost always the more common explanation is correct. Thesame is true of identifying rocks and minerals.

Metallic Luster

Suspect a sulfide first, especially if you can detect a sulfur smell. Nextmost likely, an oxide, then perhaps a metallic element or compound of one of thesemi-metals (As, Se, Bi, Te). Non-metallic luster could indicate any othergroup. However, some of the sulfide minerals are non-metallic, notablysphalerite, orpiment, realgar and cinnabar. The last three are identified bytheir bright colors.

Not many minerals can appear either metallic or non-metallic. Hematite andsphalerite 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. Nextmost likely, an oxide, then perhaps an element or compound of one of thesemi-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 mostlikely 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 suspectquartz 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 aremost likely to be carbonates, halides or sulfates.

Cleavage

Color

Color is far down the list because it is easily the least reliablecharacteristic of minerals. Color can always be due to an impurity or surfacestain. As an undergraduate, I was once asked to try to identify a hard brightblue mineral. I even had X-ray data to help. After running through all thecopper minerals with no luck, I looked at the X-ray data for all minerals andfound a perfect match with diopside. We had a common pyroxene mineral that isnormally white, but in this case was stained by copper. So always suspect thatcolor 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. Adense, gray mineral is either galena or it isn't. On the other hand, sandstonecan grade into siltstone, limestone into dolostone, gabbro into diorite. If arock is on the borderline between two types, it's usually not all that criticalwhere you place it. 

The Three Great Rock Families

Suspect a rock is of a given type if it has one or more of thesecharacteristics:

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 mechanismssimilar to sedimentary rocks. Some people classify them as volcanic, others assedimentary.


Igneous-Volcanic

Igneous-Plutonic


Metamorphic

If the original rock type can be recognized, the rock can be described byprefixing meta- to it: metaconglomerate, metarhyolite, metabasalt, etc. Oftenthis is the only way of naming the rock.


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Created 26 September 2001, Last Update 31 May 2020