Phrehnite, Beith.

Phrehnite, Beith.

Prehnite is an inosilicate of calcium and aluminium, with the formula Ca2Al(AlSi3O10)(OH)2 The crystal structure is described as stalactitic or botryoidal aggregates. This specimen, which weighs 4 Kg. is mostly a very pale green colour, and some small botryoidal crystals are noted, however the bulk of the specimen is in massive form. A small vugh as in the inset shows some small flattened "rosettes" of radiating crystals. It is not unusual for prehnite to be associated with calcite, and this specimen has several pinkish crystals of calcite, the largest of which is 6 cm. overlying the phrehnite. The location is Loanhead quarry, Beith, Ayrshire in Scotland.  

Tourmaline, rossmanite var.

Tourmaline, rossmanite var.

Tourmaline is a boron silicate mineral, which occurs as an accessory mineral in igneous and metamorphic rocks. There are a large number of varieties of tourmaline, each with a slightly different composition, resulting in a wide range of colours. Rossmanite has the formula (LiAl2)Al6Si6O18(BO3)3(OH)3OH and is  is usually described as being pink in colour, however, this specimen from Cornwall  has a grey green colour, and is composed of tightly compacted radiating sprays of prismatic crystals.  

Petrified wood.

Petrified wood.

This specimen of petrified wood, more commonly called fossil wood, or wood opal. is made of almost pure silica, SiO2 in the form of chalcedony, opal, and quartz. Minor amounts of other minerals, such as iron, copper, manganese, etc. impart a certain amount of colour. Petrification occurs when wood is buried in water saturated sediment, which greatly reduces oxygen. Mineral laden fluids slowly replace the original wood in a process called petrification,  preserving in fine detail the original structure. This specimen, although a mineral, is in reality a fossil. Origin is unknown. There is a comprehensive article on petrified wood in Silurian No. 1, which can be located elsewhere on this website.

Pyromorphite. Cumbria.

Pyromorphite. Cumbria.

Pyromorphite,    Pb5(PO4)3Cl  is a secondary mineral of lead sulphide, Pb5 This specimen is from an open cut near the upper Mexico mine, Caldbeck fells, in Cumbria, and is composed of very short, apple green, prismatic crystals. The crystals are arranged in laterally orientated connecting chains. The crystals are well defined, but quite stubby, unlike the elongated barrel type crystals from the mines of central Wales. All faces of the specimen have a little evidence of pyromorphite, but this face is by far the best populated

Baryte, origin unknown.

Baryte, origin unknown.

Baryte is barium sulphate,  BaSO4   baryte crystals can be of several different forms, tabular, massive, nodular, fibrous, or radiating. I am not keen on collecting specimens from unknown locations, but this specimen was a gift, and is included in my collection as a "type specimen", The platey crystals are a dirty grey colour, and there is a sprinkling of small chalcopyrite crystals, and two or three quartz crystals in the crevices.