Streak (mineralogy)

Definition and Importance of Streak

• Streak is the color of the powder produced when a mineral is dragged across an un-weathered surface.
• It is a diagnostic tool in mineral identification.
• Streak is particularly important for opaque and colored materials.
• It is less useful for silicate minerals.
• Minerals with no visible streak are considered to have a white or colorless streak.

Factors Affecting Apparent Color

• The apparent color of a mineral can vary due to trace impurities or disturbed crystal structure.
• Small impurities do not greatly affect the absorption of light when a mineral is crushed into powder.
• The color of the powder produced during streak testing is more consistent than the apparent color.
• Impurities can radically change the wavelengths of light reflected by the mineral.
• Streak testing breaks the specimen into randomly oriented microscopic crystals.

Streak Plate and Alternative Surfaces

• The surface across which the mineral is dragged is called a streak plate.
• Streak plates are generally made of unglazed porcelain tile.
• The unglazed underside of a porcelain bowl or vase can also be used as a streak plate.
• The back of a glazed tile can serve as an alternative streak surface.
• Sometimes comparing the streak with another streak plate can provide a more accurate description.

Mineral Hardness and Streak

• A streak can only be made by minerals softer than the streak plate.
• The Mohs scale of mineral hardness is used to determine if a mineral can produce a streak.
• Minerals with a hardness of around 7 on the Mohs scale can create a streak.
• Harder minerals require filing or crushing to determine the color of the powder.
• Most harder minerals have an unhelpful white streak.

Examples of Streak Colors

• Some minerals leave a streak similar to their natural color, such as cinnabar, lazurite, and native gold.
• Fluorite always has a white streak despite appearing in various crystal colors.
• Hematite, with a black appearance, leaves a red streak, giving it its name derived from the Greek word for blood.
• Galena, resembling hematite, can be distinguished by its gray streak.
• Streak colors can provide additional identification clues for specific minerals.

Streak (mineralogy) Data Sources