Properties of Minerals

Mineral Properties and Testing

• Minerals are naturally occurring, inorganic substances with a definite chemical composition and crystalline structure.

Streak

• Streak is the color of a mineral in its powdered form.
• It is often more consistent and reliable than the color of the mineral in its solid form.
• Testing method: Drag the mineral across a streak plate (unglazed porcelain) to produce a line of powder and observe the color of the streak.
• Example: Hematite leaves a reddish-brown streak, while pyrite leaves a greenish-black streak.

Hardness

• Hardness measures a mineral's resistance to scratching.
• It is determined using the Mohs hardness scale, which ranks minerals on a scale from 1 (talc) to 10 (diamond).
• Testing method: Perform scratch tests using reference materials of known hardness (e.g., fingernail, copper coin, steel file).
• Alternatively, try to scratch the reference material with the mineral.
• Determine where the mineral falls on the Mohs scale based on which materials it can scratch and which materials can scratch it.
• Example: Quartz has a hardness of 7 and can scratch glass, which has a hardness of about 5.5.

Cleavage

• Cleavage describes how a mineral breaks along specific planes of weakness in its crystal structure.
• Minerals with good cleavage break smoothly along these planes, creating flat, shiny surfaces.
• Testing method: Examine the mineral for natural planes of separation.
• Gently tap the mineral with a hammer or use a knife to see if it breaks along flat surfaces.
• Note the number of cleavage directions and the angles between them.
• Example: Mica exhibits perfect cleavage in one direction, allowing it to split into thin sheets, while calcite has three directions of cleavage that form rhombohedral fragments.

Luster

• Luster refers to the way a mineral reflects light from its surface.
• It can range from metallic and glassy to dull or earthy.
• Testing method: Observe the mineral under a good light source.
• Describe how the light interacts with the surface of the mineral using descriptive terms such as metallic, vitreous, pearly, resinous, or dull.
• Example: Galena has a metallic luster, while quartz typically has a vitreous luster.