Reflection and Transmission
May 14, 2008
Quartz (primarily silicon dioxide, SiO2) is the second most common mineral in the Earth’s crust after feldspar. Together, silicon (Si) and oxygen (O) make up about 75% of the Earth’s crust. Silicates, silicon-oxygen compounds, have been found on the Moon, and detected on each of the inner planets of our solar system. Some estimates say that the continental crust contains about 12% quartz. Most of the color variations in quartz are due to impurities that create different absorption bands in the rock. Geologists believe that the purple color of amethyst is due to the presence of iron, manganese or hydrocarbons. Rose quartz’s color is probably related to titanium or aluminum. There is one rare form of quartz owes its color not to lattice impurities but rather to the same source of color as the blue sky: Rayleigh scattering.
Scattering from particles that are much smaller than the wavelength of light is referred to as Rayleigh scattering. Blue light (c.475 nm) is scattered 4.3 times more strongly than red light (c.650 nm). As a result, sunlight (white sunlight contains all the colors of the spectrum) that reflects off of quartz appears bluish. In transmission, however, the absence of the scattered blue light leaves only the longer wavelengths to come through, and thus quartz appears reddish or yellowish.
The picture above shows a flake of quartz from a pegmatite in the Cape Ann Granite found in Rockport, Massachusetts. On the left is a picture in reflected (scattered) sunlight and on the right is the same flake viewed in transmitted sunlight. Note the color differences, a classic signature of Rayleigh scattering. The scattering sources in this piece of blue quartz are tiny inclusions of rutile (titanium dioxide, TiO2) or other minerals. Unlike blue quartz, other colored quartzes are the same color when viewed in transmission or reflection. Note that not all blue quartzes owe their color to Rayleigh scattering -- most have inclusions of blue minerals.