Tables for the Determination of Common Opaque Minerals
Measurements of the optical properties of opaque minerals using a reflected-light polarizing microscope are difficult for the unskilled investigator because they require demanding and exacting techniques. Unlike transparent minerals, which can be easily identified by tests using a transmitted-light petrographic microscope, opaque minerals have few optical properties that can lead to unambiguous identification. The principal properties used to identify opaque minerals are color and color intensity, but these involve subjective decisions on the part of the investigator. Color is determined in part by the colors of the surrounding minerals; color intensity is influenced by reflectance. This, in turn, is influenced by sample preparation. Many teaching institutions, private consultants, and professional companies do not possess microscopes which have the additional equipment to test microhardness, or photometers to test the reflectance of minerals; however, microhardness and reflectance data have been included for those people who have access to such equipment.
The following two tables, one for colored minerals, one for noncolored minerals, include 95 common opaque minerals, 13 transparent minerals, and 8 minerals that range from transparent-translucent to opaque. The tables are by no means complete because they are designed to serve as a rapid and efficient way of identifying the common opaque minerals. We have found by experience that unless told the identity of a mineral, the unskilled user often finds alphabetical tables of opaque mineral properties imposing and time consuming. In an attempt to overcome this problem, we use an identification scheme which enables the user to identify a mineral by a process of elimination. The scheme is straightforward and utilizes a tree diagram (Fig. 1) in conjunction with a list of the mineral properties that can be easily observed under the microscope. The mineral is identified by moving from the top of the tree diagram to the bottom through a series of decisions based on optical properties. These properties are, in order: color intensity, pleochroism, anisotropism and its intensity, internal reflection, and relative hardness. A final choice of the mineral’s identity can be made when the end of the branch is reached. In only a small number of cases are all five decisions required to make this choice. Since this procedure may not always give a unique answer, additional features are included that will help elucidate the mineral’s identity. These are: the presence or absence of cleavage and twinning, the composition of commonly associated minerals, and a summary of key identifying criteria.
Published:January 01, 1987