In this article, we will discuss the identification of minerals using streaks on a porcelain plate. In Geology, it is important to recognize minerals and rocks. Sometimes the color of different minerals is the same and it is difficult to identify them only by visual testing. However, the same color minerals have different colors in powder form (streak) and it can be used to differentiate them from each other.
What are minerals?
A mineral is a naturally occurring chemical compound, usually of crystalline form and abiogenic in origin (not produced by life processes).
A mineral has one specific chemical composition.
There are over 5300 known minerals. The silicate minerals compose over 90% of the earth’s crust. Silicon and oxygen constitute over 75% of the earth’s crust. Minerals are distinguished by various chemical and physical properties.
Differences in chemical composition and crystal structure distinguish the various species, which were determined by the mineral’s geological environment when formed.
Changes in the temperature, pressure, or bulk composition of a rock mass cause changes in its minerals.
What is Streak in Minerals?
The streak is a way of determining the color of a mineral in powdered form.
A specimen of the mineral is placed against a piece of unglazed porcelain plate and scraped across the surface of the plate. The streak of a mineral is the color of the powder produced when it is dragged across an un-weathered surface.
The streak may be completely different from the color of the hand specimen. One of the simple ways of determining the streak of a mineral is to rub a specimen across a piece of unglazed porcelain known as a streak plate.
The streak of a mineral is the color of the powder produced when it is dragged across an un-weathered 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 silicate minerals, most of which have a white streak or are too hard to powder easily.
The apparent color of a mineral 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.
What is a streak or porcelain plate for minerals?
The surface across which the mineral is dragged is called a “streak plate”, and is generally made of unglazed porcelain tile. In the absence of a streak plate, the unglazed underside of a porcelain bowl or vase 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.
Streak plates come in white and black. The default is white, but black can be handy as a second option. The great majority of minerals have a white streak. This is the streak of gypsum but resembles streaks from many other minerals.
Corundum leaves a white streak (left), but after wiping (right) it is clear that the plate itself was scratched by the hardness-9 mineral. Gold (top), platinum (middle), and copper (bottom) have characteristic streak colors, best seen on a black streak plate.
What is a 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 is known as its “streak.”
Procedure
- The streak test should be done on clean, 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.
Impotance of Streak Test in Minerals Identification
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 high specific gravity and similar color and habit.
Mineral Hardness is also used in the identification of minerals.
