Physical Properties Of Minerals II: Polymineralic Analysis

Overview

Source: Laboratory of Alan Lester - University of Colorado Boulder

The physical properties of minerals include various measurable and discernible attributes, including color, streak, magnetic properties, hardness, crystal growth form, and crystal cleavage. These properties are mineral-specific, and they are fundamentally related to a particular mineral’s chemical make-up and atomic structure.

This video examines several physical properties that are useful in field and hand sample mineral identification— color, luster, streak, hardness, magnetism, and reaction with acid. Unlike crystal form and crystal cleavage, these properties are somewhat more closely linked to mineral chemical composition than to atomic structure, but both do play a role.

It is important to recognize that rocks are aggregates of mineral grains. Most rocks are polymineralic (multiple kinds of mineral grains) but some are effectively monomineralic (composed of a single mineral). Unlike crystal form and cleavage, which are terms reserved for mineral specimens, geologists might on occasion refer to a rock as having a general sort of color, hardness, magnetism, or reaction with acid. In other words, the physical properties looked at here are potentially appropriate for use with rocks as well as with specific minerals.

Procedure

Preparation

In order to observe and analyze the physical properties of minerals as is done in this video, there are a few preparatory steps that should be taken. First, collect a group of mineral samples. Suggested samples include hematite, magnetite, calcite, dolomite, and galena. Establish a surface for examining the specimens. A clean table-top is suitable, perhaps with a piece of white paper on the table surface. Obtain a porcelain streak plate, a hardness kit, magnet and compass, and dilute HCl (2-5%).

Application and Summary

Historically, the evaluation of the physical properties of minerals has been a key first step in mineral identification. Because microscopic and modern analytical instrumentation (e.g. petrographic microscopy, x-ray diffraction, x-ray fluorescence, and electron microprobe techniques) are not available in the field, recognition and use of observed physical properties can be important diagnostic tools.

Evaluating and observing the physical properties of minerals is an excellent means to demonstrate how the macroscopic features