Bryn y Rafr mine, West Wales. Typical brecciated material from this site, which is a mix of host rock, Quartz, and Sphalerite. The host rock is a fine grained dark mudstone. Brecciation is the result of inwardly exploding walls of deep fractures when there is a sudden decrease of pressure in the hot mineral rich fluids which are circulating through the fracture.
Quartx, SiO2, Ystrad Meurig quarry, Ysbyty Ystwyth, West Wales. Dirty white prismatic crystals of milky quartz in a cluster, growing at random angles to the base rock which is a very roughly formed quartz aggregate. Some of the crystals taper slightly toward the base. Milky quartz is not coloured by other minerals, but is coloured by microscopic inclusions of gas or liquid incorporated during the growth process.
Siderite, Iron cabonate, FeCO3. Siglenlas mine, Nr. Llanidloes, Central Wales Orefield. There is very little left in the way of spoil heaps at this location, so to find this particular specimen was extremely fortunate. The rock is brecciated with pockets filled with siderite. One face is completely covered with siderite, with a bunch of well formed crystals in the centre. The crystals, even if they are very weathered, make identification much easier.
Marcasite, FeS2, and Quartz, SiO2, Bryn y Rafr mine, west Wales. This brecciated rock is a mixture of host rock, quartz, and marcasite. Unfortunately marcasite is prone to pyrite decay, and this is well illustrated on this specimen. The marcasite has broken down resulting in an encrustation of sulphur. The process has been treated and almost stopped by storing in an airtight box and surrounded with a liberal supply of silica desiccant pouches. The rock itself is very fragile with cracking apparent, probably because of marcasite content. The same can be said of the quartz, which is also very fragile.