Biotite mica. Cornwall.

Biotite mica. Cornwall.

Biotite is classified  as a mineral, but is actually a common group of phyllosilicate minerals with the approximate formula of  K(Mg,Fe)3AlSi3O10(F,OH)2   This specimen is simply labelled as from from Cornwall, but after investigation of it's appearance, it is very similar to other specimens collected from Trelavour downs, St.Dennis, Cornwall which have been described as having radiating, foliated, reddish brown mica sheets. If my assumption is correct, it probably also has a lithium, Li, content,and can be called Lithium, Biotite mica.

Obsidian "Apache tear", Superior, Colorado.

Obsidian "Apache tear", Superior, Colorado.

Obsidian is a volcanic glass classed as a mineraloid, and is composed of approximately 70 % Silica, SiO2, This Apache tear variety is formed as as the result of the differential cooling of siliceous, rhyolitic lava flows. The grey mineral surrounding the obsidian is perlite, an amorphous volcanic glass. The name Apache tear is attributed to an Apache legend. Apaches were fighting U.S. cavalry on a mountain overlooking Superior in Arizona. With no hope of winning they rode their horses off the mountain, and their families shed tears which turned to stone on hitting the ground. There are six towns named Superior in the U.S.,and this specimen comes from Superior, Colorado.

Baryte on calcite. Isle of Sheppey.

Baryte on calcite. Isle of Sheppey.

Baryte, BaSO4 Barium, sulphate. Specimens such as this one are very distinctive, and easily recognisable as coming from the Isle of Sheppey. The Isle is largely formed from the London clay. Within the clay are found septarian nodules coated with yellowy brown calcite resembling marzipan, upon which are perched radiating crystals of baryte. Even though there has been much research, the process of how they are formed is still a mystery.

Fluorite. Egremont.

Fluorite. Egremont.

Fluorite is calcium fluoride, CaF2  It may be called fluorspar, and is a halide mineral. In a pure form Fluorite is colourless and transparent, however it is an allochromatic mineral and is very variable in colour because of the inclusion of trace elements. This specimen is light purple in colour and is a slab of well formed and intergrown cubic crystals on a limestone matrix. The location is loosely given as Egremont  in Cumbria.

Astrophyllite, Kola peninsula.

Astrophyllite, Kola peninsula.

Astrophyllite is a highly complex mineral, a hydrous, potassium, iron, titanium, silicate. with a formula of (K,Na)3 (Fe++,Mn)7 Ti2 Si8 O24 (O,OH)7 It is usually found in the form of a bladed stellate aggregate, often in a radiating form. ( astron from the Greek for star, and pyllon, also from the Greek for leaf ). It is not a common mineral and is found in only a few locations, for instance this specimen came from the Kola peninsula in Russia.