Chalcedony, erratic.

Chalcedony, erratic.

Chalcedony. is a microcrystalline form of silica,  SiO2.  The silica is in two forms, quartz, and moganite, both of which have the same formula, but different crystal structures. The specimen, known as banded chalcedony,  is composed of alternating layers of grey and cream deposits of chalcedony, topped with an encrustation of small quartz crystals.  The crystals are somewhat battered, because this specimen is an erratic that has been transported by an ice sheet. The location is only known as the Lincolnshire fells.

Epidote.

Epidote.

Epidote is  a calcium aluminium iron sorosilicate  mineral, with the formula  Ca2Al2(Fe3+;Al)(SiO4)(Si2O7)O(OH). It is a rock forming mineral of secondary origin, occurring in marble and schistose rocks of metamorphic origin. It may also occur as a product of the alteration of the usual minerals found in igneous rocks, for instance, micas, amphiboles, feldspars, pyroxenes.  The randomly orientated crystals in this specimen are prismatic, and are very dark green, almost black in colour. It was collected from Mount Painter, in South Australia, by the club co-founder Jim Nicholls, while working at Woomera.

Brochantite, Australia.

Brochantite, Australia.

Brochantite is a sulphate mineral, Cu4SO4(OH)6, often referred to as a secondary mineral of copper. The encrustation on this specimen is a mix of brochantite and another copper secondary mineral, probably malachite in the form of minute botryoidal clusters ( see insets).  The brochantite crystals are prismatic, and very small, scattered here and there throughout the specimen, and are a very slightly darker green.  This Australian specimen was collected in the 1960's by our club co-founder, Jim Nicholls, while he was working there on rocket technology.  

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.