3.2.2 Read the text and fulfill the after-reading exercises. (r.P – 4.3, 4.6) Physical Properties

Physical properties of minerals are often the only guidelines for field identification of specimens that may occur combined with other minerals or in very small amounts in a particular sample.

1. Minerals grow into consistent crystal shapes, which are controlled by the chemical composition of the minerals. There are six crystal systems that include all minerals. Some mineral compounds can form more than one mineral with entirely different crystal structures.

Some examples of the crystal form are:

• isometric,

• elongated,

• flattened.

2. How a mineral resists scratching is the measure of its hardness. A mineral will resist scratching or demonstrate its hardness relative to the substance used to scratch it or the substance that the mineral can scratch. German mineralogist Friedrich Mohs developed a hardness scale involving ten minerals from the softest to the hardest during the 19th century. His scale, known as Mohs hardness is the standard by which mineral hardness is determined. Only fresh mineral surfaces should be tested for hardness.

Hardness	Mineral	Test
1	Talc	Can be scratched by a fingernail
2	Gypsum	Can be scratched by a fingernail
3	Calcite	Can be scratched by a penny
4	Fluorite	Can be scratched by a glass plate
5	Apatite
6	Orthoclase	Can be scratched by a knife blade
7	Quartz
8	Topaz	Can be scratched by a steel file
9	Corundum
10	Diamond	Hardest known mineral

Table 3 Mohs Hardness Scale

3 . Color is a physical property of limited value in mineral identification. For example, quartz can be of nearly any color because of slight mineral impurities. Quartz can be cloudy, white, pink, purple, red, black, smoky, orange and brown. Color is not usually sufficient by itself to identify a mineral; however, in combination with other physical properties it can assist in mineral identification.

4. The streak of a mineral is its color in the powered form. Streak is obtained by drawing the mineral across an unglazed porcelain plate. In many minerals the streak is more diagnostic than color and is often very different from the mineral color. Minerals that are harder than the porcelain plate produce no streak.

5. Minerals have different weights relative to each other. Some are relatively heavy and others are light. Specific gravity of a mineral is its weight relative to the weight of an equal volume of water. It is obtained by dividing the weight of the mineral in air by the difference between its weight in air and its weight in water. Most minerals specimens are difficult to measure for specific gravity because they are usually associated with other minerals. However, very heavy or very light minerals are most easily recognized by specific gravity.

6. How a mineral reflects light determines its type of luster. Most minerals are considered to have metallic or nonmetallic luster, with a few having a submetallic luster.

Fig. 17. Branching formation of bright metallic gold crystals (Photo by John Bett)