File:Emeralds in pegmatitic granite 8 (37992559234).jpg

Emeralds in pegmatitic granite

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 over 5500 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).

Beryl is a beryllium aluminosilicate mineral, Be3Al2(Si6O18). It has a nonmetallic luster, forms sharp, hexagonal crystals, is very hard (H=7.5 to 8), and can be any color. A frequently encountered color is pale bluish-green. Beryl has a glassy luster and no obvious cleavage

Transparent beryls are gemstones. The gem name depends on the color: - deep green = emerald - bluish = aquamarine - pink = morganite - rich yellow = golden beryl - red = bixbite - yellowish-green to pale greenish = heliodor - clear/colorless = goshenite

Emerald is green, chromiferous beryl. The large emerald crystals shown here are in pegmatitic granite, which is a very coarsely-crystalline, intrusive igneous rock. In the matrix, the whitish-gray is potassium feldspar (KAlSi3O8 - potassium aluminosilicate), a common and essential mineral in granite. The dark gray is quartz (SiO2 - silica), also a common and essential mineral in this lithology. Pegmatites are usually form during the final phases of crystallization of a cooling pluton. The large crystal size in pegmatites is due to cooling of a relatively water-rich magma. The magma from which pegmatites form is usually enriched in noncompatible elements. This results in the formation of scarce minerals, such as beryl (= the emeralds seen here) and tourmaline (which is present on the flip side of this rock).

Photo gallery of beryl: <a href="https://www.mindat.org/gallery.php?min=819" rel="noreferrer nofollow">www.mindat.org/gallery.php?min=819</a>

Photo gallery of emerald: