Calcite, calcium carbonate, CaCO3. This is one of several specimens collected from Cregiau quarry, near Pentyrch in south Wales. A scattering of milky white crystals, mostly nailhead crystals, with a number of rhomboid crystals, are sitting on a bed of light brown calcite. The bed of brown calcite is in a form which is hard to describe, but seems to be rosettes made up of small bladed crystals growing in a haphazard arrangement.
This specimen is from Bryn y Rafr mine, on the shore of Nant y Moch reservoir, west Wales. It is a loose breccia, with a number of voids, and is typical from this mine. It has components of quartz, marcasite, and wall rock. The quartz is very fragile, and the marcasite is suffering from pyrite decay, which is leaving a deposit of sulphur. The marcasite is of a granular nature as seen in the inset picture. The pyrite decay is occurring, even though the specimen is kept in a sealed box, with a considerable number of dessicant sachets.
This specimen of breccia is from Henfwlch mine, Nant y Moch reservoir, Ceredigion. Angular clasts of wall rock are embedded in a matrix mainly composed of a mineral in the Ankerite/Dolomite series. The brown colour suggests that it is a manesium rich/iron defictent ankerite which is also referred to as ferroan dolomite. This type of specimen is plentiful at this site.
The specimen is composed of two minerals, sapphirine and anthophyllite. Sapphirine is a silicate of magnesium and aluminium, (Mg,Al)8(Al,Si)6O20. The mineral is formed in high temperature metamorphic rock, and the crystal colour is light to dark blue. Anthophyllite Mg2Mg5Si8O22(OH)2 is an amphibole mineral, with a fibrous crystal habit, and may be classed as an asbestos mineral. the origin of this specimen is from the Malene Supracrustal Volcanics, about 100Km ESE of Godthaab/Nuk, West Greenland.
Quartz, silicon dioxide, SiO2 is the most abundant mineral in the earth's surface. Geodes form in gas pockets in volcanic rock, or rounded cavities in sedimentary rock. The process begins with the formation of a hard shell which lines the cavity. followed by subsequent deposits of minerals from circulating hydrothermal fluids. This specimen has a hard outer shell, and a deposit of quartz crystals within the shell. There is a very slight purple hue on some of the crystals deeper inside the geode, which has been broken on both ends. The smaller inset picture is of the rear of the geode. The origin of the specimen is only known to have come from Morocco.