Not uncommonly, some soil-forming minerals will exhibit optical properties that are apparently inconsistent with their crystal form and structure; and inconsistent with properties listed above and inmost reference texts. Such misalliance of characteristics are due to one or a combination of factors:

• Strain or mechanical distortion of the crystal structure which may produce birefringence in some isotropic minerals and some uniaxial minerals to appear biaxial (e.g., quartz and calcite in metamorphic rocks).

• Variations in chemical composition. Depending on the environment of formation, many minerals can have isomorphic substitution of various elements in the crystall structure which can alter the optical properties. This chemical variation is due to the fact that structurally equivalent positions in the crystallographic framework may be occupied by ions of different elements. The ability of one ion to proxy for another is governed by: a) ionic radii of the tow ions; b) temperature of crystallization; and, c) crystal structure. Many minerals belong to groups having a solid solution series between two end members, making it often difficult to distinguish a particular member. This effectis often observed by differences in color, change in refractive index, an dtwinning pattern.

• Crystal size. Some mineral species can occur in such fine-grained particles that aggregates appear isotropic (e.g., calcite in limestone, pedogenic carbonates, quartz in shale). Most clay minerals are too small to identify in the petrographic microscope.

• Color. Minerals that are colored (biotite, hornblend) under plane-polarized light, will under cross-polarized light, display interference tints that result from the combination of the mineral color (due to absorption) and the interference color. Thus, interference colors observed through the microscope may not match the sequence of colors in the order listed in color charts.

• In some twinned minerals (e.g., orthoclase) the grain may be weathered such that only a single twin is observed. In such instances, the determination of optic sign or interference figure may be required for mineral identification.

• Alteration due to physical or chemical weathering. Many minerals may be physically rounded during transport such that crystal faces are no longer present. Chemical weathering may obsure the identity of th eoriginal grain. However, the evidence of physical or chemical weathering may provide useful information.

• Some minerals may form pseudomorphs after other minerals due to replacement. A pseudomorph is a crystal having the outward form of another mineral, which is has replaced by substitution. Examples include calcite or quartz after gypsum; quartz may also form pseudomorphs after calcite.

• Artifacts in the thin section or grain mounts may look like mineral material and show optical properties similar to minerals. Artifacts include air bubbles, grinding powder, lint fibers, some organic residues, and strained mounting resin which may cause birefringence. Although not necessarily an artifact, some minerals such as quartz may be fractured during sawing and preparing a thin section. This may be due to poor preparation techniques or using an old or imporperly cooled cutting blade. Grain fracturing may look like physical or chemical weathering.

This page (http://www.ces.ncsu.edu/plymouth/programs/minerals.html) created by
Vera MacConnell, Research Technician, I on December 1, 1997.
Last Updated on December 8, 1997.