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The Streak Test for Minerals


Done by scraping the mineral across an unglazed porcelain streak plate.


streak test

The Streak Test: Marks, known as "streaks," are produced by scraping mineral specimens across unglazed porcelain plates. On the left, a specimen of pyrite has produced a black streak. On the right, a specimen of rhodochrosite has produced a white streak. Many minerals produce a white streak, and some geologists prefer using a black streak plate for these minerals because the mineral particles in the streak are easier to observe. This photo by Ra'ike is used here under a GNU Free Document License.

What is the Streak Test?

The "streak test" is a method used to determine the color of a mineral in powdered form. The color of a mineral's powder is often a very important property for identifying the mineral.

The streak test is done by scraping a specimen of the mineral across a piece of unglazed porcelain known as a "streak plate." This can produce a small amount of powdered mineral on the surface of the plate. The powder color of that mineral known as its "streak."


Streak Colors of Common Minerals

Andalusite White or colorless (hardness is about the same as the streak plate).
Anhydrite White.
Apatite White.
Arsenopyrite Dark grayish black.
Augite White to greenish gray. Augite can be splintery and close to the hardness of the streak plate, so brittle fragments, rather than a powder, will sometimes be produced.
Azurite Light blue.
Barite White.
Bauxite White. Often discolored to pink, brown, or red by iron staining.
Benitoite White.
Beryl Colorless (harder than the streak plate).
Biotite White to gray (don't be deceived by flakes).
Bornite Grayish black.
Calcite White.
Cassiterite Colorless.
Chalcocite Grayish black.
Chalcopyrite Greenish black.
Chlorite Greenish to greenish-black to white.
Chromite Dark brown.
Chrysoberyl Colorless (harder than the streak plate).
Cinnabar Red.
Clinozoisite White.
Copper Metallic copper red.
Cordierite Colorless (harder than the streak plate).
Corundum Colorless (harder than the streak plate).
Cuprite Brownish red.
Diamond Colorless (harder than the streak plate).
Diopside White to light green.
Dolomite White.
Enstatite White to gray.
Epidote White or colorless (about the same hardness as the streak plate).
Fluorite White.
Fuchsite White (often sheds tiny green mica flakes).
Galena Lead gray to black.
Garnet Colorless (harder than the streak plate).
Glauconite Dull green.
Gold Metallic gold yellow.
Graphite Black.
Gypsum White.
Halite White.
Hematite Red to reddish brown.
Hornblende White. Brittle, often leaves black cleavage debris behind instead of a streak.
Ilmenite Black.
Jadeite Colorless (harder than the streak plate).
Kyanite White or colorless (about the same hardness as the streak plate in one direction).
Limonite Yellowish brown.
Magnesite White.
Magnetite Black.
Malachite Green.
Marcasite Grayish Black.
Molybdenite Bluish gray, grayish black.
Monazite White.
Muscovite White, often sheds tiny cleavage flakes.
Nepheline White.
Nephrite Colorless (harder than the streak plate).
Olivine White or colorless (about the same hardness as the streak plate). Often sheds tiny granules instead of a powder.
Orthoclase White.
Plagioclase White.
Prehnite White.
Pyrite Greenish black to brownish black.
Pyrophyllite White.
Pyrrhotite Grayish black.
Quartz Colorless (harder than the streak plate).
Rhodochrosite White.
Rhodonite White.
Rutile Pale brown.
Scapolite White.
Serpentine White.
Siderite White, very light brown.
Sillimanite White or colorless (about the same hardness as the streak plate).
Silver Silvery white.
Sodalite White or light blue.
Sphalerite White to yellowish brown, often with an odor of sulfur.
Spinel Colorless (harder than the streak plate).
Spodumene White or colorless (about the same hardness as the streak plate).
Staurolite Colorless (harder than the streak plate).
Sulfur Yellow.
Sylvite White.
Talc White to pale green.
Titanite White.
Topaz Colorless (harder than the streak plate).
Tourmaline Colorless (harder than the streak plate). Specimens often fracture, shedding small particles.
Turquoise White, greenish, bluish.
Uraninite Brownish black, grayish.
Witherite White.
Wollastonite White.
Zircon Colorless (harder than the streak plate).
Zoisite White.

How to Conduct the Streak Test

The streak test should be done on clean, unweathered, or freshly broken specimens of the mineral. This is done to reduce the possibility that a contaminant, weathered coating, or tarnish will influence the results of the test.

The preferred method for conducting a streak test is to pick up a representative specimen of the mineral with the hand that you write with. Select a representative point or protrusion on the specimen that will be scraped across the streak plate. With your other hand, place the streak plate flat on a tabletop or laboratory bench. Then, while holding the streak plate flat and firmly in place on the tabletop, place the point of the specimen firmly against the streak plate, and, while maintaining firm pressure, drag the specimen across the plate. Now examine the streak to determine its color and to confirm that it is a powder, instead of grains, splinters, or broken pieces.




Don't Be Wimpy!

The most common error made by people who are doing the streak test for the first time is to lightly rub the specimen back and forth on the surface of the streak plate. This will not produce a proper streak. Some mineral specimens are so hard that very firm pressure and determination are required to produce a mineral powder.



Why Use the Streak Test?

The streak test is valuable because many minerals occur in a variety of apparent colors - but all specimens of that mineral share a similar streak color. For example: specimens of hematite can be black, red, brown, or silver in color and occur in a wide variety of habits; however, all specimens of hematite produce a streak with a reddish color. This is a valuable test for hematite. It can be used to differentiate hematite from a large number of other opaque minerals with a high specific gravity and similar color and habit.

Fluorite is another mineral where the apparent color can be different from the color of the streak. Specimens of fluorite can be green, yellow, purple, blue, or colorless. However, all specimens of fluorite have a white streak. Specimens of pyrite always have a brassy yellow color; however, all specimens of pyrite produce a black streak.

Related:   The Acid Test

Don't Be Deceived!

A number of things can cause a streak test to give unreliable results. To avoid problems, keep the following items in mind.

Refreshing Your Streak Plate

Streak plates that have been used heavily will be covered with streaks and powdered mineral. They can easily be cleaned with water and a piece of wet or dry 220 grit sandpaper. Aluminum oxide or silicon carbide sandpaper works best because the granules are hard enough to smooth the surface of the streak plate. The sanding should be done wet to control dust.

Mineral collection

The best way to learn about minerals is to study with a collection of small specimens that you can handle, examine, and observe their properties. Inexpensive mineral collections are available in the Geology.com Store.

Other Uses for Streak Plates

In addition to their use in doing the streak test, streak plates can be used any time you need a small amount of powdered mineral. In doing the acid test to distinguish calcite from dolomite, dolomite might require being powdered to show effervescence with dilute hydrochloric acid. Simply use the streak plate to make some powder of your specimen and add acid to it right on the streak plate. For this test, a black streak plate makes observation easier because powdered dolomite is white.

A few minerals will produce an odor upon being broken or powdered. For example, sphalerite releases an odor of sulfur when it is broken or powdered. Scraping it across a streak plate is a convenient way to conduct this test.

Hints to other mineral properties can be obtained while doing the streak test. Minerals harder than the streak plate are quickly identified. Experienced testers can estimate the hardness of a specimen by how difficult it is to mark the streak plate. Olivine often reveals its granular nature, augite often reveals its splintery cleavage, and black tourmaline often reveals its brittleness. When you do the streak test, look for more than the color of a specimen's powder.

More Minerals
  Diamond
  Rock, Mineral and Fossil Collections.
  Minerals
  Quartz
  Fluorescent Minerals
  Calcite
  Topaz
  Mineral Identification Chart

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