Minerals are identified by their physical properties. How would you describe th
Minerals are identified by their physical properties. How would you describe the mineral making up the sample in Figure 1.2? You might say that it is shiny, gold, and shaped like cubes. Each of these characteristics is a physical property.
Physical properties can vary within the same minerals, so caution should be applied when identifying minerals based on any one property. Colour is an example of a property that is not a very helpful diagnostic tool in many cases, because some minerals, such as quartz, can come in a variety of colours (Figure 1.3).
Nevertheless, colour can occasionally be helpful, as in the case of olivine, which comes in shades of olive green (Figure 1.4). This overview covers each of the key physical properties in detail to prepare you to identify the minerals in your kit.
Streak
As you have seen in Figure 1.3, the colour of a mineral specimen is not necessarily a helpful diagnostic property. What may surprise you is that when minerals are powdered, the colour of the powder is consistent between different specimens of a particular mineral, even if the specimens themselves appear to be very different colours. This makes the powder colour very useful for identifying the mineral.
A mineral’s streak is the colour of the powder mark left behind when the mineral is rubbed on an unglazed piece of porcelain called a streak plate (Figure 1.5). Notice that the streak of pyrite in Figure 1.5 is dark grey, even though the pyrite is a dull gold colour.
Lustre
Lustre refers to how a mineral’s surface reflects light. In general, lustre can be described as metallic or non-metallic. Minerals with a metallic lustre shine brightly, are opaque, and often have the colour of a metal, such as silver, gold, copper, or brass (Figure 1.6). If a mineral with a metallic lustre becomes tarnished, then a submetallic (or semi-metallic) lustre results.
While minerals with a metallic lustre are often shiny, not all shiny minerals are metallic. Bright and shiny minerals can be adamantine or glassy (vitreous). Minerals might have a waxy, silky, or pearl lustre, and some are dull (earthy). Figure 1.7 shows examples of these lustres, but it doesn’t include all of the possibilities. Minerals could have a resinous lustre, or even a greasy lustre!
Crystal Form & Mineral Habit
The geometric shape that a mineral naturally grows into is its crystal form. Crystal form reflects the orderly internal arrangement of atoms within the mineral. If minerals have space to grow when they are developing, they will display their crystal form (e.g., Figure 1.8). Ideal growth conditions do not always occur, however. If a mineral encounters another crystal, it will continue to grow according to the same internal atomic arrangement as before, but it will have to follow that pattern within the available space. so many minerals do not display their ideal crystal form due to crowded conditions during growth. Examples of crystal form are shown in Figure 2.8.
Mineral habit addresses the fa