Dolomite. CaMg(CO3)2 is an anhydrous carbonate mineral, sometimes referred to as dolostone. It has several industrial uses including the production of float glass, and as an additive to soil and potting mixtures as a pH buffer. This specimen is a tangled collection of tabular crystals with curved faces, which is the normal habit for dolomite . The source of this specimen is from one of the limestone/dolomite quarries in Sligo, Ireland.
Tourmaline is a crystalline boron silicate mineral, with 33 different minerals in the group. The most common mineral is schorl, which is a sodium iron end member. Tourmaline is one of the most chemically complicated minerals, so I have chosen not to show the formula. The specimen of quartz has quite a number of schorl inclusions, mostly with the same alignment, and the base of the quartz has an area of iridescence. It is also noted that the quartz is divided by an opaque layer of unknown mineralogy about 1 mm. thick, which the schorl crystals penetrate. Unfortunately the origin is only listed as Brazil.
Calcite, CaCO3 Calcium carbonate. There is nothing very special about this specimen, except that it is one of the first specimens I collected many years ago. It is a collection of scalenahedral crystals in a limestone vugh. A few of them on the periphery are slightly larger, and all of them have a dusty coating probably derived from the presence of iron. I collected the specimen from Blaengwynlais quarry, Tongwynlais, Near Cardiff in South Wales
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.
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.