Gypsum, altered.

Gypsum, altered.

Gypsum (CaSO4.2H2O)  calcium sulfate dihydrate is an evaporite mineral which can ocurr in many different colours, and crystal habits. The most common colour is  white or off white, and the most common crystal habit is in the form of selenite. This specimen collected from Barry harbour in South Wales in 2010. was lying on the beach, hence the light covering of algae. It has come from bands in the red marls in the cliff face. The original nodule has been altered by water passing through the rocks, which has left this crenulated specimen with nearly all the exposed surfaces coated with an alteration product of small scalenahedral crystals of calcite.

Rutile, Australia.

Rutile, Australia.

Rutile is titanium oxide, TiO2 It is a common accessory mineral in high-temperature and high-pressure metamorphic rocks and in igneous rocks. There is a wide colour variation, with the golden variety most sought after for decorative purposes.  The crystal form is usually slender and acicular. The crystals in this specimen are not very well defined, are short and stubby, and almost black. It was collected from Mount Crawford, South Australia.

Molybdenite.

Molybdenite.

Molebdynite is molybdenum disulfide, MoS2  It was at one time thought to be lead, and has a similar appearance. The small metallic crystals in this specimen are scattered throughout the quartz. It's origin is unknown.

Stilbite with Actinolite

Stilbite with Actinolite

The main component of this specimen is stilbite, ( the orangey mineral ) with some actinolite, ( the dark mineral ) and a minor amount of laumantite.   Stilbite, which is a serial name, is a zeolite, NaCa4Al9Si27O72 · 28(H2O) and actinolite is a very complex amphibole silicate mineral, with the formula Ca2(Mg,Fe)5Si8O22(OH)2   The white mineral which appears as a crust, with only a few minute crystals visible, is thought to be limontite, another zeolite mineral.  The specimen was collected by the club co-founder Jim Nicholls from Arkaroola bore, South Australia.  

Flint.

Flint.

Flint, sometimes referred to as chert by geologists, is a cryptocrystalline form of quartz,  SiO2  Very often it is found in chalk deposits as nodules. These specimens which I collected from Sandringham warren in Norfolk interested me because of their non-nodular appearance. The one in particular seems to be a cemented collection of small irregular shaped pieces of flint. The location is from a footpath cutting through a small area of higher ground which exits the warren. Pieces of flint are eroding from the narrow cutting through which the footpath has been cut.