Obsidian. Mexico.

Obsidian. Mexico.

Very often referred to as "volcanic glass" Obsidian is neither a rock or a mineral. It is classed as a mineraloid because it is not crystalline. Odsidian is formed in rhyolitic eruptions, and cools so quickly that crystals do not have time to form. Its composition is generally 70% of  SiO2    The three specimens are from Mexico, The larger one is colloquially known as Snowflake obsidian. The snowflake appearance is due to inclusions of spherulites in the form of clusters of radiating crystals of cristobalite. The brown specimen is coloured by trace amounts of iron.

Quartz, stressed. Llandinam, Powys.

Quartz, stressed. Llandinam, Powys.

These specimens of quartz,  SiO2  were collected on the top of one of the highest hills in my locality. The hill is between Llandinam and Newchapel and is the site of a wind farm. The specimens were in a pile of material excavated to accommodate the concrete base for a turbine. The quartz is unusual for it's form, with the appearance of having been twisted by being subjected to pressure . The hill is 590 m 0r 1935 ft ASL, and is Silurian, Landovery,  in age.

Malachite, Katanga prov. Congo.

Malachite, Katanga prov. Congo.

Malachite is a copper carbonate mineral with the formula  Cu2CO3(OH)2 and is formed from the weathering of other copper ores. It is a mineral that is always pleasing to the eye. It can be found in several forms, Botryoidal, Massive, Stalactitic, and fibrous as is this specimen.The radiating fibrous clumps are very silky in appearance and seem to branch out towards the point of termination. The specimen is from the Miringa mine in Katanga ( Shaba )  province, Congo.  

Hemimorphite.

Hemimorphite.

Hemimorphite is a sorosilicate mineral with the formula Zn4Si2O7(OH)2-H2O  It was at one time named Calamine, but confusion with another mineral led to the new name of hemimorphite, which is a reference to it's hemimorphic crystal habit. It occurs in two major forms, Clear or white thin bladed crystals, or green/blue botryoidal encrustations. This specimen is the botryoidal type, set in an iron rich matrix, and is from Santa Maria del Orto, Durango, Mexico.  

Sphalerite, Driggeth, Cumbria.

Sphalerite, Driggeth, Cumbria.

Sphalerite, zinc sulfide,( Zn,Fe)S  from the Driggeth mine in Cumbria, is the most common ore of zinc This specimen has a number of crystals with a very untidy  tetrahedral habit. Also on the specimen,  are dozens of minute pyrite cubes,which can only be seen under high magnification.