Basic Mineral Descriptions: How to Read a Rock

Author: 约翰史密斯

  Hello, everyone. This article introduces some common terms for describing mineral properties.
  Why learn to describe minerals in the first place? The Earth contains an enormous variety of mineral deposits. Specimens that look completely different may be the same mineral, while specimens that look almost identical may have entirely different compositions.
  Judging a specimen by appearance alone can easily lead us astray. Take the famous “fool’s gold,” for example:

  It may glitter like gold, but it is simply pyrite and has nothing to do with gold itself. Even the broad category of iron ore includes hematite, magnetite, limonite, and many other minerals. Describing a specimen systematically makes it much easier to compare it with other samples and identify what it actually is.
  A useful mineral description covers several properties:

  • Color: What is its overall color? Is it uniform or a mixture of colors?

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

  • Luster: Does it have a metallic or waxy appearance? Does it sparkle?

  • Cleavage: Does it split along straight, flat planes? Are those planes 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: This one is self-explanatory.

  • Hardness: This one is self-explanatory.

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

  • Magnetism: This one is self-explanatory.

  • Optical activity: Does a thin section rotate the plane of polarized light? If so, by how much?

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

  • Environment: Was the mineral found in igneous, metamorphic, or sedimentary rock? Was there anything unusual about the setting?