Minerals that cannot be determined by a streak plate typically include those that are harder than the streak plate itself, such as diamonds and corundum, as they will not leave a streak. Additionally, minerals that are non-metallic and have a very light color or are transparent may not produce a visible streak. Furthermore, minerals that are powdery or have a very fine texture might not yield reliable streak results.
Those minerals that are harder than the unglazed porcelain streak plate will scratch it rather than leave a streak.
Topaz (8), corundum (9), and diamond (10).
Mineral hardness measures a mineral's resistance to scratching, while streak is the color of the powdered form of a mineral. Hardness is determined by the Mohs scale, while streak is identified by rubbing the mineral across a porcelain plate to see the color left behind.
Diamond and quartz are colorless minerals that also have a colorless streak. This means that when these minerals are scratched against a white ceramic plate, they leave behind a streak that is also colorless.
A streak test is performed by scraping a mineral across a porcelain streak plate, which is unglazed and usually white. The mineral leaves a colored streak on the plate, which can help identify the mineral, as different minerals produce distinct streak colors regardless of their external appearance. The streak should be observed in a consistent lighting condition to ensure accurate results. This test is particularly useful for distinguishing between minerals with similar physical characteristics.
Hematite, gold, feldspar or magnetite
Two minerals that do not leave a streak on a streak plate are quartz and fluorite. Both minerals have a hardness higher than that of the streak plate, so they will not leave a streak when rubbed against it.
Streak color is determined by scraping the mineral across a a streak plate, (which is made of unglazed porcelain), and then observing the color of the streak, which is left on the plate. Note that some minerals do not leave a streak, as they are too hard. Thus, it is important to learn other identification methods, to use in conjunction with streak color, in order to identify minerals.
Those minerals that are harder than the unglazed porcelain streak plate will scratch it rather than leave a streak.
Gabbro does not have a streak because it is a coarse-grained rock made up of minerals like plagioclase feldspar and pyroxene that are harder than the streak plate. As a result, gabbro does not leave a streak when rubbed on a streak plate.
A query to mindat.org, a mineral database, revealed almost 100 minerals listed with a streak of brown. Two fairly familiar minerals named were schorl and chromite.
Topaz (8), corundum (9), and diamond (10).
A streak test is not used to identify minerals with a hardness greater than 7 on the Mohs scale, as these minerals can scratch the streak plate. Additionally, streak tests may not be effective for identifying minerals that have a streak color similar to the streak plate itself.
Very hard minerals like diamonds can leave no streak because their hardness prevents them from being scratched by the streak plate. Since the streak test involves rubbing the mineral against a surface to leave a streak, extremely hard minerals won't leave a streak behind because they are tougher than the testing material.
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Streak is the property of a mineral that is determined by rubbing the mineral on a special plate to reveal the color of its powdered form. This can help identify minerals because a mineral's streak color is often different from its external color.
Mineral hardness measures a mineral's resistance to scratching, while streak is the color of the powdered form of a mineral. Hardness is determined by the Mohs scale, while streak is identified by rubbing the mineral across a porcelain plate to see the color left behind.