Rhyolite: Gemstone Jewelry

Materials Information: Rhyolite

Makeup:

Rhyolite is a silicate, meaning that it is composed of silicon and oxygen. It is formed when water with silicon, oxygen and other elements is deposited on a surface. When the water evaporates, the other elements harden and are left behind. The color of the rock formed depends on which "other" elements are present. The major minerals found in rhyolite are quartz and feldspar

Hardness:

Rhyolite tends to vary greatly in hardness, depending on the specific components of the sample, but it tends to measure around a 6 on the Moh's Hardness Scale. This means that the stone is between the hardness of a typical knife blade and a steel file.

http://www.galleries.com/minerals/hardness.htm

What is Rhyolite?

Rhyolite is a volcanic igneous rock that has two main forms. Most of the rhyolite found is porphyritic, meaning that larger crystals are embedded in other very fine crystals. The resulting circular patterns are due to the varied cooling rates of the magma and phenocrysts (larger crystals). The second type of rhyolite, banded rhyolite, is formed by layers slowly building up over time.

What is igneous rock?

Igneous rock forms when magma (molten rock) solidifies. When solidification occurs underground, the rocks that are formed are called intrusive rocks (granite). Magma can also solidify on the surface, forming extrusive rocks (basalt, obsidian, rhyolite).

Because rhyolite is an extrusive igneous rock, it is also known as a volcanic rock. Once the magma flows on to the surface, it is known as lava. Silica present causes rhyolitic lava to be viscous. Because it cools quickly and is thick, the lava does not flow the way people normally think of for a volcanic eruption. Instead, the rhyolite solidifies on top of itself and tends to form a lava dome. Rhyolitic lava can also erupt so violently that it collapses upon itself to form a caldera (depression)

Dome: http://volcanoes.usgs.gov/images/pglossary/LavaDome.php

Caldera: http://volcano.oregonstate.edu/education/vwlessons/volcano_types/caldera.htm

Why does volcanic rock vary from glassy (like obsidian) to rough (like basalt)?

Molten rocks (magma) form different textures depending on the rate of cooling. Magma that cools quickly allows for very small crystals to form. The glassy surface of obsidian demonstrates how crystals that too small for the eye to see will create a smooth surface. As the cooling rate decreases, the crystals that are formed become larger. These larger crystals produce a rock with a rougher surface.

What are phenocrysts?

Phenocrysts are relatively large crystals which are embedded in an igneous rock. In rhyolite, the phenocrysts are often quartz and sanidine.

Where is rhyolite found?

Rhyolite is generally found in the interiors of continents. Many sites in our National Parks include areas of rhyolite. There is even a town in Death Valley, Nevada that was named Rhyolite because of the amount of the stone they came across in their mines. Among other places, rhyolite can be found in deposits in Yellowstone National Park in Wyoming. Some of the silica in rhyolite is dissolved by hot water flowing through geysers in the park. The silica at the surface forms rock that seals the water in until it builds to too great a pressure.

http://www.nps.gov/deva/historyculture/rhyolite-ghost-town.htm

http://www.nps.gov/yell/naturescience/hotspring.htm

Why is rhyolite often called the volcanic equivalent to granite?

Rhyolite and granite form in similar ways, although granite is an intrusive rock (formed underground) and rhyolite is an extrusive rock (formed above ground). Magmas containing silica form granite when they crystallize deep beneath the Earth's surface.

Wasn't there a satellite named Rhyolite?

Rhyolite was apparently the code name for a satellite surveillance program which was used to monitor microwave transmissions by the Soviet Union. The project was renamed Aquacade when the name Rhyolite became public as part of the trial of Christopher John Boyce and Andrew Daulton Lee (“the Falcon and the Snowman”) in 1981.

http://www.planet4589.org/space/misc/names.html

http://www.historycommons.org/context.jsp?item=civilliberties_217

http://www.astronautix.com/craft/rhyolite.htm

http://www.justice.gov/marshals/history/boyce/index.html

Sources:

Rock and Gem: The definitive guide to rocks, minerals, gems and fossils. Ronald Louis Bonewitz.

http://us.dk.com/nf/Book/BookDisplay/0,,9780756633424,00.html

National Audobon Society Guide to Rocks and Minerals.

http://www.randomhouse.com/catalog/display.pperl?isbn=9780394502694

Other Websites

Scientific information about the mineral rhyolite:

http://hyperphysics.phy-astr.gsu.edu/hbasees/Geophys/rhyolite.html