Arsenopyrite

Arsenopyrite

Arsenopyrite is an iron arsenic sulphide  FeAsS.  With a content of 40 to 45% arsenic, it is the principal ore of arsenic. The specimen has inclusions of chalcopyrite on a different face to the one illustrated. The specimen is from Hingston Down Quarry in Cornwall, which is a site noted for the mineral arthurite, another arsenic mineral.  

Orbicular Rhyolite

Orbicular Rhyolite

In highly silicified deposits of rhyolitic tuff, quartz and feldspar crystallise in a manner which creates the orbicular inclusions. This specimen was described on aquisition as "leopardskin jasper ", but on further investigation I discovered that leopardskin or orbicular jasper, is in reality orbicular rhyolite, which is an extrusive igneous rock. Because it is used in jewellery, it may be that jasper sounds better than rhyolite, when being advertised by traders. This specimen is from Aquas Calientes, Chihuahua, Mexico, and should really be listed in my rocks collection, rather than as a mineral.

Calcite "dogtooth"

Calcite "dogtooth"

Calcite,   CaCO3   This specimen of "dogtooth "calcite is from the Santa Eulalia mining district, Chihuahua, Mexico. The crystals are grey in colour, many with lustrous surfaces, which implies colouration by impurities rather than surface coating. There is only one damaged crystal, the cross section of which shows interesting markings. Many of the dog tooth crystals are coated with very small irregular crystals of calcite, and the rear of the specimen is about 50% covered with microscopic druzy calcite.

Sphalerite

Sphalerite

Sphalerite. ( Zn,Fe ) S ),  Zinc sulphide, with variable iron content. Sphalerite is the main ore of zinc, and sometimes referred to by miners as zinc blende, or black jack. This specimen is an agglomerate of loosely cemented pieces of rock with nearly all of the surface, and all crevices, coated with small crystals of sphalerite . The crystals on the rear of the specimen are slightly larger than those on the front. The location is from the "Second Sun" vein, Nentsberry Haggs mine, Cumbria.  

Gypsum, selenite var. south Australia

Gypsum, selenite var. south Australia

Gypsum, CaSO4 . 2H2O is an evaporite mineral. This selenite variety specimen is composed of numerous sprays of small tabular crystals, varying in colour from clear to light green. The location is from the Pernatty lagoon, near to Mount Gunson copper mine in South  Australia. The green colour is attributed to copper from the nearby mine.