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MINERAL PROPERTIES and identification procedures 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 is 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. Streak is very useful in distinguishing two minerals with the same color but different streak. A fine example where streak is very useful is gold (yellow streak), and chalcopyrite (black streak). Most light colored, nonmetallic 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 and cost-effective 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 a 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.
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