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Macroscopic Characteristics of Minerals Part 1: Luster and Color
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Macroscopic Characteristics of Minerals Part 1: Luster and Color

Introduction to Mineral Identification

In this section, we will learn about the importance of luster in identifying minerals and the two categories of luster: metallic and non-metallic.

Luster and Mineral Identification

  • Luster is related to the speed that light travels through a mineral, or its refractive index.
  • Minerals that transmit light slowly, with a refractive index greater than 3, generally have a metallic luster.
  • Metallic minerals are opaque with a mirror-like, shiny luster.
  • Non-metallic minerals tend to be less shiny than metallic minerals and can range from opaque to transparent.
  • There are seven types of non-metallic lusters: adamantine, vitreous, pearly, silky, resinous, greasy, and dull.

Types of Non-Metallic Lusters

Adamantine Luster

  • Minerals with adamantine luster are transparent to translucent and are the shiniest of the non-metallics.
  • Diamonds are the most well-known example of adamantine luster.

Vitreous Luster

  • Vitreous luster looks like a freshly cleaned pane of glass.
  • These minerals are transparent to translucent and have a low refractive index.
  • Quartz and calcite have a vitreous luster.

Pearly Luster

  • Pearly luster resembles the subtle iridescence found in natural pearls.
  • Iridescence occurs when light reflects off different crystallographic planes as it passes through a crystal.
  • Muscovite is an example of a mineral with pearly luster.

Silky Luster

  • Minerals with silky luster are composed of thin parallel fibers making them reminiscent of silk.
  • Ulexite and gypsum are examples of minerals with silky luster.

Resinous Luster

  • Resinous luster resembles hardened maple syrup or frozen pine resin.
  • Sphalerite can sometimes be found in nature with a resinous luster, along with metallic or adamantine lusters.

Greasy Luster

  • Minerals with a greasy luster appear as though they are covered in a thin layer of oil and also feel greasy.
  • Graphite is an example of a mineral with greasy luster.

Dull Luster

  • Minerals with dull or earthy luster are the least shiny of them all, as light is primarily scattered at the surface instead of reflected.
  • Kaolinite is an example of a mineral with dull luster.

Coloration in Minerals

  • Minerals typically get their coloration from the presence of transition metals.
  • Transition metals absorb specific wavelengths of visible light, which affects the color that is reflected off a mineral’s surface into your eye.
  • Allochromatic minerals only contain trace amounts of transition metals.

Idiochromatic Minerals

This section explains how idiochromatic minerals get their color and how it can be useful for identifying them.

Color Sources

  • Idiochromatic minerals are "self-colored" by the presence of a transition metal in their standard chemical formula.
  • Other sources of color include structural defects in the crystal lattice.

Streak Test

  • The streak test is one of the ways that color can be useful for identifying a mineral.
  • The streak is the powdered residue left behind after rubbing a mineral on a piece of unglazed porcelain.
  • Most non-metallic minerals have a white streak, while metallic minerals tend to have dark-colored streaks.

Examples

  • Hematite always has a reddish-brown streak, regardless of its color.
  • Calcite always has a white streak, regardless of its color.
  • Pyrite always has a dark gray streak, while gold has a pale-yellow streak.

Luster and Cleavage

This section explains what luster and cleavage are and how they can be used to identify minerals.

Luster

  • Luster refers to the way light interacts with the surface of a mineral.
  • Metallic luster describes minerals that look like metals, such as gold or silver. Non-metallic luster describes all other types of luster, including vitreous (glassy), pearly, silky, waxy, greasy, and dull.

Cleavage

  • Cleavage refers to the way that some minerals break along planes of weakness.
  • Minerals can have perfect, good, poor, or no cleavage at all.

Examples

  • Mica has perfect cleavage in one direction, which means it breaks into thin sheets.
  • Quartz has no cleavage and breaks with a conchoidal fracture, which means it breaks into curved surfaces like broken glass.
Video description

Now that we know what minerals are, how can we identify them? Geologists use a number of properties of minerals to figure out which mineral they are looking at. The first two we will examine are luster and color. Let's get a closer look! Script by Jared Matteucci Watch the whole Geology playlist: http://bit.ly/ProfDaveGeo Mathematics Tutorials: http://bit.ly/ProfDaveMath Classical Physics Tutorials: http://bit.ly/ProfDavePhysics1 General Chemistry Tutorials: http://bit.ly/ProfDaveGenChem Biology Tutorials: http://bit.ly/ProfDaveBio Microbiology Tutorials: http://bit.ly/ProfDaveMicrobio Botany Tutorials: http://bit.ly/ProfDaveBotany Zoology Tutorials: http://bit.ly/ProfDaveZoo EMAIL► ProfessorDaveExplains@gmail.com PATREON► http://patreon.com/ProfessorDaveExplains Check out "Is This Wi-Fi Organic?", my book on disarming pseudoscience! Amazon: https://amzn.to/2HtNpVH Bookshop: https://bit.ly/39cKADM Barnes and Noble: https://bit.ly/3pUjmrn Book Depository: http://bit.ly/3aOVDlT