Question

The "Streak" of a mineral refers to which physical characteristic?

• A. The brilliance or shine of the surface
• B. The color of the mineral in powder form
• C. The tendency to break along planes
• D. The relative density of the mineral

Answer 1

Answer:

B. The color of the mineral in powder form

Mineral Streak: A Diagnostic Physical Property

Definition:

• The streak of a mineral is defined as the color of its powder when it is rubbed against an unglazed porcelain plate (a streak plate).
• This is a crucial physical characteristic used in mineral identification, often proving more reliable than the mineral's apparent color.

Why Streak is Important for Identification:

• Consistency: While the external color of a mineral specimen can vary due to impurities or weathering, its streak is typically constant for a given mineral.
• Distinguishing Similar-Looking Minerals: Many minerals share similar external colors but have distinct streaks. For example:
  • Hematite can appear black, silver, or red, but its streak is always a reddish-brown.
  • Pyrite (Fool's Gold) is brass-yellow and has a greenish-black streak.
  • Magnetite is black with a black streak.

How to Determine Streak:

1. Obtain a mineral sample and a streak plate (unglazed porcelain or ceramic tile).
2. Firmly rub the mineral across the surface of the streak plate, applying moderate pressure.
3. Observe the color of the fine powder or line left behind.

Factors Affecting Streak:

• Hardness: Minerals softer than the streak plate will produce a colored streak. Minerals harder than the streak plate will not leave a streak or will only produce a white scratch from the plate itself.
• Composition: The chemical composition of the mineral directly determines the color of the powdered particles.

Common Examples:

• Quartz (various colors) has a white streak.
• Galena (silvery-gray) has a gray streak.
• Sulfur (yellow) has a pale yellow streak.

Distinction from External Color:

• It is vital to differentiate the streak color from the mineral's overall appearance. Surface alterations, inclusions, or different crystalline forms can lead to variations in external color that do not reflect the mineral's true streak.