What is streak of a mineral

Small amounts of an impurity that strongly absorbs a particular wavelength can radically change the wavelengths of light that the specimen reflects, thereby changing the apparent color.

Dragging the specimen to produce a streak breaks into randomly oriented microscopic crystals, reducing the small impurities' effect. The surface whereover the mineral is dragged is called a " streak plate " and generally is made from unglazed porcelain tile. Instead of actually crushing a mineral to determine the streak, it is much simpler 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. 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. Many common minerals of different colors produce a white streak when crushed and powdered.

The crushed powder of a number of common minerals appears lighter to darker gray, or even black. The most interesting and important result of a streak test is when the color of the powdered sample differs in color from the color of the bulk specimen.

This happens in only a few of the common minerals. An example is pyrite, which typically is brassy-yellow, but the streak is black. The determination of the color of a mineral sample would best be done by instrumental measurement, such as a spectrometer. We'll simplify this seven color spectrum even more by dropping indigo. A grayscale varies continuously from white to black.

Materials Needed. Streak plate. A streak plate is made of fused silica. The streak plate surface is a bit bumpy. A streak plate has a hardness of about 7 on the Mohs scale. Assume the sample's hardness is in fact less than that of a streak plate, about 7 on the Mohs scale.

Performing a streak test correctly requires enough vertical force exerted on the sample to crush a sharp corner or edge against the streak plate to produce a powder. If insufficient force is exerted, one might incorrectly conclude that the sample is harder than a streak plate about 7 on the Mohs scale. Materials for the streak test. It has proven to be a powerful property because it is generally very consistent from specimen to specimen for a given mineral.

Two minerals that have similar outward color may have different colors when powdered. For instance, the minerals hematite and galena can be confused when both have a gray color. However, hematite's streak is blood-red, while galena's streak is lead gray. Hematite pictured above is probably the most well known example of streak with its completely surprising streak color.