Classification of Minerals

How Rock Forming Minerals are Identified in Hand Samples

Sep 30, 2009

Geologists use many mineral properties to identify minerals and better understand the rocks they are studying.

Minerals are made from the bonding of different elements in different environments. For this reason, minerals take on different properties that allow them to be visually identified. Even within one kind of mineral, different crystal shapes will emerge depending on the conditions under which the mineral formed.

Mineral Properties

There are 91 chemical elements that occur naturally on Earth. Every mineral contains one or more of these elements and every mineral contains either a single element or a compound of elements. The bonds that hold together these element compounds also create unique mineral properties that aid in their identification. These properties can be identified in hand samples, but when too small to view with the naked eye, can also be viewed under a petrographic microscope.

Crystal Symmetry and Habit

Crystal symmetry refers to a mineral's complex geometric form. Crystal symmetry is most easily identified in a perfect mineral specimen, however this is rare when a mineral forms within a rock. There are 32 possible combinations of crystal symmetry and these are referred to as crystal classes. Fortunately, only 12 of these crystal classes are represented by common minerals.

Habit refers to the common and characteristic crystal class that a mineral assumes as it grows. For example, the habit of the mineral pyrite (Fool's Gold) is to have a cubic crystal form, but it can also have a octahedron crystal form.

Cleavage, Fracture and Twinning

Cleavage describes how a mineral breaks along a certain plane and is dependent on the orientation of the mineral's internal atomic bonds. A mineral's tendency to cleave along one plane more easily than another is evidence of the bonds being weaker in one direction than another.

The fracture of a mineral describes the appearance of a mineral that has been broken in a direction other than the direction of cleavage. For example, quartz, which has virtually no cleavage in any plane, has the appearance of a conchoidal fracture when it is broken.

Twinning is a property that develops while a mineral is growing and describes crystal aggregates. Much like conjoined twins, mineral twinning occurs when two or more crystal faces grow together. This is very common in feldspars and quartz.

Hardness and Specific Gravity

The hardness of a minerals is one of the most useful properties when identifying minerals in that it is easy to test for. The hardness refers to a minerals resistance to scratching. The Mohs' Hardness Scale is used to identify the hardness of a mineral and employs simple tools for testing. For example, a human nail can scratch any mineral ranked three or lower on the scale, such as calcite and gypsum. Diamond, the hardest substance on Earth, is ranked 10 on the scale.

The specific gravity of a mineral is the ratio of its density to the density of water. A laboratory balance can be used to measure the specific gravity, but if the sample is a hand sample, one can simply measure the specific gravity by the weight of the specimen compared to other mineral specimens of similar size.

Color and Luster

The color of a mineral is easily identifiable, even in small grain samples. Olivine, for example, is distinctly green. Other minerals, such as quartz, can take on multiple colors. It is colorless in its pure form, but due to some compositional variations, it can also be black, brown, pink, green, purple or opalescent blue. When identifying the color of a mineral is difficult, one can also look at the color of the mineral in powder form. This is most easily achieved by scraping the mineral against a piece of unglazed porcelain and observing the color of the powder streak left behind on the porcelain.

Luster refers to whether or not the mineral has a metallic look to it. A mineral will either have a metallic or a non-metallic luster. Pyrite, for example, has a metallic gold luster, which is why it is also called Fool's Gold. Non-metallic lusters can be described as pearly, waxy or dull.

Source:

Dietrich, Richard V. and Skinner, Brian J., Rocks and Rock Minerals. NY: John Wiley and Sons, 1979.

The copyright of the article Classification of Minerals in Minerals is owned by Alexandra Matiella Novak. Permission to republish Classification of Minerals in print or online must be granted by the author in writing.