MINERAL PROPERTIES:
STREAK
Streak is the color of a crushed mineral's powder. The color of a
mineral's powder may differ from the actual color of the mineral. This property
can be useful
for mineral identification.
Almost every mineral has an inherent streak color, no
matter what color the actual mineral is. For example, Calcite occurs in many different
colors, shapes, and varieties. But every single variety of Calcite has a white streak.
A streak is useful in distinguishing two minerals with the same color but different
streak. A good example is distinguishng Gold (yellow streak), and Chalcopyrite (black streak).
Most light colored, non-metallic minerals have a white or
colorless streak, as do most silicates, carbonates, and most transparent minerals.
The streak test is most useful for identifying dark colored minerals, especially metals.
How to use streak as an identification
mark
When testing for streak, the mineral must be crushed to
determine the color of its powder. The color of the powder is the color of the streak.
Instead of actually crushing a mineral to determine the streak, it is much simpler to swipe the mineral across a streak plate. A streak plate is an unglazed
piece of porcelain, such as the underside of a ceramic tile. This is the most popular
method of streak testing, since the color of the streak plate is white, the color of the
mineral trace is easy to see. For minerals that are harder than the streak plate, this
test cannot be used, since the mineral will remove tile material. This is rarely a
problem, though, since most minerals where this test will be significant are softer than
the streak plate (the streak plate has an average hardness of about 6½ on the Mohs scale).
Most mineral references don't make a distinction
between a white and colorless streak, since the difference is minimal. A mineral with a
white or colorless streak will not leave a visible streak on a streak plate.
How to test using streak
Swipe the mineral against the streak plate, but not hard
enough to damage it. The color of the powder that results on the streak plate is the
streak color.
