Question

Which physical characteristic refers specifically to the color of a mineral in its finely powdered form?

• A. Luster
• B. Fracture
• C. Streak
• D. Specific Gravity

Answer 1

Answer:

C. Streak

Mineral Streak

Definition:

• The color of a mineral in its powdered form is known as its streak.

• This is determined by grinding a mineral against an unglazed porcelain plate, called a streak plate.

Importance in Mineral Identification:

• While the external color of a mineral can vary due to impurities or weathering, its streak is generally a constant and reliable characteristic.

• For example, hematite can appear black, silver, or earthy red, but its streak is always a reddish-brown.

• Pyrite, often called "fool's gold" due to its brassy yellow color, has a greenish-black streak, which helps distinguish it from real gold (which has a yellow streak).

Comparison with External Color:

• The external color of a mineral is due to the way it absorbs and transmits light, which can be affected by surface conditions.

• The streak, however, represents the true color of the mineral's pigment.

Streak Plate:

• A streak plate is typically made of unglazed porcelain and has a hardness of about 7.

• Minerals softer than the streak plate will leave a streak. Minerals harder than the streak plate will scratch the plate instead of leaving a streak.

Example Characteristics:

1. Hematite: External color varies, but streak is always reddish-brown.

2. Pyrite: External color is brassy yellow, but streak is greenish-black.

3. Magnetite: External color is black, streak is black.

4. Galena: External color is metallic gray, streak is gray.