The Basics of Mineral Description: How to Read a Rock

Author: 约翰史密斯

  Hello, everyone. Today we will introduce some common terms used to describe the many properties of minerals.

  First, why do we need to learn how to describe minerals? The Earth contains an extraordinary variety of mineral deposits. Some specimens that look completely different are actually the same mineral, while others that look remarkably alike have entirely different compositions.

  Consider what happens if we judge a specimen only by its most obvious appearance. Take the famous “fool’s gold,” for example:

  It may glitter with a golden-yellow shine, but it has nothing to do with gold: it is simply pyrite. And even among iron ores, there are hematite, magnetite, limonite, and many others. A systematic description of a mineral therefore helps us compare specimens later and determine what mineral each one actually is.

  To describe a mineral, we need to examine it from many angles:

• Color: What color does it appear overall? Is the color uniform or mixed?

• Transparency: Is it transparent, translucent, or opaque?

• Luster: Does it look metallic or waxy? Does it sparkle?

• Cleavage: Does it split along straight, flat surfaces? Are those surfaces continuous or discontinuous?

• Fracture: What shape does the broken surface have? Is it conchoidal or rough? Does it follow a regular pattern?

• Twinning: Is the crystal twinned? If so, what type of twinning does it exhibit?

• Density: Self-explanatory.

• Hardness: Self-explanatory.

• Streak: What color is the powder produced by rubbing the mineral, and does it have any distinctive properties?

• Magnetism: Self-explanatory.

• Optical activity: Does a thin section rotate the plane of polarized light? What is the period of that rotation?

• Fluorescence: Does it glow under ultraviolet light or at a particular wavelength?

• Environment: In what setting was the mineral found—in igneous, metamorphic, or sedimentary rock? Was there anything unusual about that environment?