File:Amethyst geode (Serra Geral Formation, Lower Cretaceous; southeastern Brazil) 1 (32934451701).jpg

Amethyst geode from Brazil. (public display, Montana Bureau of Mines and Geology Mineral Museum, Butte, Montana, USA)

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are about 5400 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The silicates are the most abundant and chemically complex group of minerals. All silicates have silica as the basis for their chemistry. "Silica" refers to SiO2 chemistry. The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4. Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens "belong" to each silicon. The resulting formula for silica is thus SiO2, not SiO4.

The simplest & most abundant silicate mineral in the Earth's crust is quartz (SiO2). All other silicates have silica + impurities. Many silicates have a significant percentage of aluminum (the aluminosilicates).

Quartz (silicon dioxide/silica - SiO2) is the most common mineral in the Earth's crust. It is composed of the two most abundant elements in the crust - oxygen and silicon. It has a glassy, nonmetallic luster, is commonly clearish to whitish to grayish in color, has a white streak, is quite hard (H≡7), forms hexagonal crystals, has no cleavage, and has conchoidal fracture. Quartz can be any color: clear, white, gray, black, brown, pink, red, purple, blue, green, orange, etc.

Purple quartz is called amethyst. The coloring agent for amethyst is not agreed upon. Some workers say that it is due to Fe+4 impurity, some say the impurity is Fe+3, and others say it is Mn.

The amethyst geode shown above was hosted in a lava flow in Brazil. Geodes are small to large, subspherical to irregularly-shaped, crystal-lined cavities in rocks. They form when water enters a void in a host rock and precipitates crystals. The most common geode-lining mineral is quartz.

From museum signage: Cracks and fissures formed on the Earth's surface when Gondwanaland, a supercontinent, separated about 142 million years ago. Through these fissures, massive basalt flows developed, known as the Serra Geral Formation in southeast Brazil, covering an area of about 1.2 million square kilometers.

This geode began as a gas bubble in hot lava. As the lava cooled and eventually hardened, the bubble left a void in the rock. Sometime later, hot water seeped through the lava and deposited quartz, a form of silica, into the void. The geode's outer layer is made of chalcedony (microcrystalline quartz), followed by clear quartz, and then by the beautiful amethyst quartz crystals. The purple color is derived from iron found in the crystal matrix.

Today, miners are removing the geodes from the weathered basalt through horizontal adits. Used solely for ornamental purposes, the abundant amethyst geodes are the most important mineral deposit within the Serra Geral Formation.

Stratigraphy of host rocks: Serra Geral Formation, Lower Cretaceous

Locality: unrecorded/undisclosed site in southeastern Brazil

Photo gallery of quartz and amethyst <a href="http://www.mindat.org/gallery.php?min=3337" rel="nofollow">www.mindat.org/gallery.php?min=3337</a> and