Color in Minerals: The Science of Mineral Color
Color is the first thing most people notice about a mineral, and it is one of the most complex of all physical properties. The color of a mineral arises from specific interactions between visible light and the electrons in the mineral's crystal structure. Understanding these interactions requires some knowledge of quantum mechanics and solid-state physics, but the basic principles are accessible and illuminate a fascinating dimension of mineralogy.
The most fundamental distinction in mineral color is between idiochromatic and allochromatic coloration. Idiochromatic minerals derive their color from chemical components that are essential to their identity — the color-causing element is part of the mineral's defining chemical formula. Malachite (Cu2CO3(OH)2) is always green because copper, an essential component, is responsible for the color. Azurite (Cu3(CO3)2(OH)2) is always blue for the same reason. Almandine garnet (Fe3Al2(SiO4)3) is always red to red-brown because iron is essential. For idiochromatic minerals, color is a reliable diagnostic property.
Allochromatic minerals derive their color from trace impurities that are not part of the defining chemical composition. The same mineral can display radically different colors depending on which trace elements are present. Corundum (Al2O3) is colorless in pure form but is ruby when it contains chromium, blue sapphire when it contains iron and titanium, yellow sapphire when it contains iron alone, and pink sapphire when it contains lesser amounts of chromium. Quartz (SiO2) is colorless when pure, purple amethyst when irradiated aluminum is present, yellow citrine when iron is in a specific oxidation state, and smoky quartz when aluminum and natural radiation create lattice defects. For allochromatic minerals, color is an unreliable diagnostic property.
Color centers are lattice defects — misplaced atoms, vacancies, or trapped electrons — that absorb specific wavelengths of visible light. Amethyst color results from an irradiation-induced color center involving aluminum impurities. Smoky quartz color arises from a similar mechanism involving silicon vacancies. Color centers are sensitive to heat: heating amethyst destroys the color center, converting it to yellowish citrine (a commercial process used to produce much of the citrine sold today). Natural citrine, formed by volcanic heat acting on amethyst deposits, occurs in Brazil's Rio Grande do Sul state.
Pleochroism describes the property of some minerals to show different colors when viewed along different crystallographic directions in polarized light. This occurs because anisotropic minerals interact differently with light vibrating in different directions. Dichroic minerals show two colors; trichroic minerals show three. Tanzanite (blue zoisite) is famously trichroic — appearing blue, purple, or greenish-yellow depending on the viewing direction. Tourmaline, iolite, and alexandrite are well-known pleochroic gems. Gemstone cutters must orient pleochroic stones carefully to display the most desirable color.
Fluorescence is the emission of visible light by a mineral when exposed to ultraviolet (UV) radiation. When UV photons are absorbed by the mineral, electrons are raised to higher energy states. When they return to their ground state, they emit photons of lower energy (visible light). The energy difference appears as the characteristic fluorescence color. The most famous fluorescent mineral locality is Franklin and Sterling Hill, New Jersey, where the assemblage of willemite (green), calcite (red), and franklinite (non-fluorescent) produces one of nature's most spectacular light shows. Many minerals that appear ordinary in daylight reveal brilliant colors under UV. Fluorite, scheelite, calcite, sodalite, wernerite, and many others fluoresce. The property was named after fluorite, one of the most reliably fluorescent minerals.
Phosphorescence is the continued emission of light after the UV source is removed — the mineral "glows in the dark." Phosphorescent minerals include some specimens of willemite, sphalerite, and hackmanite (a variety of sodalite). Hackmanite also displays tenebrescence — it darkens when exposed to UV light and fades back to pale pink in ordinary light, a reversible photochromic effect.
Thermoluminescence is the emission of light when a mineral is gently heated (not to incandescence). Some fluorites, calcites, and feldspars emit light when warmed by hand or a hot lamp. This property results from trapped electrons (from natural radiation) being released as the mineral is heated. Calcite from certain localities exhibits brilliant blue-white thermoluminescence.