January 18, 2015
A dense, opaque variety of chalcedony. Jasper is most often red in color but can also be yellow, brown, green, or gray.
For this week’s Geology Word of the Week post, we’re going to learn a little about silica, aka silicon dioxide or SiO2. More specifically, we’re going to learn about silica minerals. Silicon and oxygen are the two most common elements in the Earth’s crust and are found in many, many minerals. In fact, silicate minerals comprise ~90% of Earth’s crust. Silica minerals are silicate minerals with the chemical formula SiO2.
The most common silica mineral (and the second most common mineral in Earth’s crust, after feldspar) is quartz. Quartz is most commonly clear or opaque white in color but can also be purple (amethyst), pink (rose quartz), yellow (citrine), and brown or black (smoky quartz). The different colors of quartz are caused by impurities and crystal defects. Quartz is one of a few varities of crystalline SiO2. Under most pressure and temperature conditions present at Earth’s surface and in Earth’s crust, quartz will crystallize as the SiO2 mineral. However, under different temperature and pressure conditions, such as deep in Earth’s mantle or at a meteorite impact site, SiO2 can form as other minerals, such as cristobalite, coesite, tridymite, or stishovite.
When SiO2 does not form with a large crystal structure but rather forms with a microcrystalline or cryptocrystalline structure, a mineral known as chalcedony forms. Like quartz, chalcedony comes in different varieties. Jasper is one common type of chalcedony. Another common type of chalcedony is agate, which has alternating clear and opaque banding. Other types of chalcedony include carnelian, chrysoprase, heliotrope, and onyx.
Here are a few more pictures of jasper rocks, courtesy of Ben Chorn:
And here’s a pretty agate that’s sitting on a shelf in my living room:
And here’s some quartz crystals on a rock that I picked up during a recent hike in South Africa: