The streak (also called powder color) of a mineral is the color of the powder produced when it is dragged across a unweathered surface. Unlike the apparent color of a mineral, which for most minerals can vary considerably, the trail of finely ground powder generally has a more consistent characteristic color, and is thus an important diagnostic tool in mineral identification. If no streak seems to be made, the mineral's streak is said to be white or colorless. Streak is particularly important as a diagnostic for opaque and colored materials. It is less useful for silicates, most of which have a white streak and are too hard to powder easily. Minerals are natural compounds formed through geological processes. ... Color is an important part of the visual arts. ... In chemistry, a silicate is a compound containing an anion in which one or more central silicon atoms are surrounded by electronegative ligands. ...

The apparent color can vary widely because of trace impurities or a disturbed macroscopic crystal structure. Small amounts of an impurity that strongly absorbs a particular wavelength can radically change the wavelengths of light that are reflected by the specimen, and thus change the apparent color. However, when the specimen is dragged to produce a streak, it is broken into randomly oriented microscopic crystals, and small impurities do not greatly affect the absorption of light. Quartz crystal Copper(II) sulfate and iodine crystal Synthetic bismuth crystal Insulin crystals Gallium, a metal that easily forms large single crystals A huge monocrystal of potassium dihydrogen phosphate grown from solution by Saint-Gobain for the megajoule laser of CEA. In chemistry and mineralogy, a crystal is a solid... Quartz crystal Copper(II) sulfate and iodine crystal Synthetic bismuth crystal Insulin crystals Gallium, a metal that easily forms large single crystals A huge monocrystal of potassium dihydrogen phosphate grown from solution by Saint-Gobain for the megajoule laser of CEA. In chemistry and mineralogy, a crystal is a solid...

The surface across which the mineral is dragged called a "streak plate," and is generally made of unglazed porcelain tile. In the absence of a streak plate, the underside of a porcelain bowl or vase, the surface of an electrical fuse, or the back of a glazed tile will work. Sometimes a streak is more easily or accurately described by comparing it with the "streak" made by another streak plate. Fine China redirects here. ...

Because the trail left behind results from the mineral being crushed into powder, a streak can only be made of minerals softer than the streak plate, around 7 on Mohs scale of mineral hardness. In this case, the color of the powder can be determined by filing or crushing with a hammer a small sample, which is then usually rubbed on a streak plate. Most minerals that are harder have an unhelpful white streak. The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. ...

Some minerals leave a streak similar to their natural color, such as cinnabar and azurite. Other minerals leave surprising colors, such as fluorite, which always has a white streak, although it can appear in purple, blue, yellow, or green crystals. Hematite, which is black in appearance, leaves a red streak which accounts for its name, which comes from the Greek word "haima," meaning "blood." Galena, which can be similar in appearance to hematite, is easily distinguished by its gray streak. Cinnabar, sometimes written cinnabarite, is a name applied to red mercury(II) sulfide (HgS), or native vermilion, the common ore of mercury. ... // Fresh, unweathered stalactitic azurite crystals showing the exceptionally deep blue of unaltered azurite. ... Fluorite (also called fluor-spar) is a mineral composed of calcium fluoride, CaF2. ... Hematite (AE) or haematite (BE) is the mineral form of Iron(III) oxide, (Fe2O3), one of several iron oxides. ... Galena is a lead ore. ...

References

• A.C. Bishop, A.R. Woolley, and W.R. Hamilton. Cambridge Guide to Minerals, Rocks and Fossils, second edition. Cambridge: Cambridge University Press, 1999. Page 12-13
• Martin Holden. The Encyclopedia of Gemstones and Minerals. New York: Facts on File, 1991. Page 251.
• Walter Schumann. Minerals of the World. New York: Sterling, 1992. Page 16-17.

External link

• Physical Characteristics of Minerals, at Introduction to Mineralogy by Andrea Bangert
• What is Streak? from the Mineral Gallery
• Streak at CSU Pomona Geology