Although it would never occur to many people to check a mineral’s streak, streak is sometimes a key diagnostic property. It is not a useful property for identifying most silicates, but is especially useful for distinguishing oxide and sulfide minerals. The streak of a mineral is the color it has when finely powdered. For mineral identification, it is much more reliable than mineral color, and it is easy to determine. The usual method of determining streak is to rub the mineral against a ceramic streak plate or other piece of unglazed ceramic. Figure 3.48 shows a red streak from hematite, and the photo below (Figure 3.49) shows a yellow streak from sulfur. Mineralogists routinely use streak when identifying minerals, both in the laboratory and in the field, but it cannot be determined for minerals harder than the hardness of a streak plate.
Steak color is a good diagnostic property because when a mineral is finely powdered, structural and other nonchemical effects are minimized. Calcite, for example, comes in many different colors, but calcite’s streak is always white. Pyrite (fool’s gold) is yellow but has a dark-colored streak, as does chalcopyrite. Gold, which has a color similar to pyrite’s (in hand specimen), has a yellow-gold streak.
Streak can be extremely useful for telling dark-colored minerals apart, especially metallic ones. For example, hematite may be red, gray, or black in hand specimen and may or may not have a metallic luster. It always, however, has a diagnostic red streak that helps distinguish specular hematite from galena, or other kinds of hematite from similarly colored minerals.
Unfortunately, most minerals have a white or colorless streak. Yet, streak is a key property for identifying a small number of minerals. The table below lists some minerals that often have diagnostic streak colors.
|Examples of Minerals that Do Not Have a White Streak|
pale or light brown
yellow brown to ocher yellow
white to light brown or yellow
very pale blue to white
very pale blue to gray or tan
pale yellow to yellow
light yellow to yellow
orange or reddish yellow
dark red to scarlet
rust red to blood red