Epidote on marine shoreline clast from Massachusetts, 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 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).

Epidote is a fairly common calcium aluminum iron hydroxy-silicate mineral, Ca2Al2Fe(Si2O7)(SiO4)O(OH), but it's actually a garbage-can mineral - it can have lots of things thrown in. Epidote has a nonmetallic luster and is typically "pistachio green", but it can be other shades of green. It is sometimes close to black-colored. It has a hardness of about 6 on the Mohs Hardness Scale. Epidote crystals are prismatic & usually short and stubby. Crystal faces are often deeply striated, like tourmaline. It has one cleavage plane. Epidote is a low-temperature mineral formed by early alteration and metamorphism of plagioclase feldspar, pyroxene, and amphibole (mafic minerals). It usually occurs along fractures and faults. If hot water moves through fractures, the adjacent country rock will be altered.

The epidote on this rock sample has fault slickenlines.

Locality: ocean shoreline at Northeastern University's marine station ("Marine Science Center"), southeastern end of the Nahant Peninsula, northeast of Boston, eastern Massachusetts, USA

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