Dolomite / Quartz

Dolomite / Quartz

Penstrowed quarry, Newtown, Central Wales. This specimen is very similar to another which was collected from a quarry at Cefn Coch, several miles away. Both quarries are in the Penstrowed grits fm, a mid Silurian sedimentary deposit. The rock has several vughs of quartz crystals, and dolomite crystals, both of which are very sharp and well formed.

Marcasite / Quartz

Marcasite / Quartz

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.

Siderite.

Siderite.

Siglenlas mine, Nr. Llanidloes, Central Wales ore field. 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.

Hydraulic brecciation.

Hydraulic brecciation.

Castell mine, West Wales. This fine example of hydraulic brecciation is made up of host rock, quartz, and ferroan dolomite. The brecciation is the result of inwardly exploding walls in deep fractures. A sudden decrease of pressure causes shards of wall rock to be injected into the hot mineral rich fluids which are circulating through the fracture. Larger pieces of the ferroan dolomite can easily be found on the spoil heaps at this long abandoned mine. Along with quartz, the ferroan dolomite would be considered as one of the gangue minerals at the mine.

Sphalerite and siderite

Sphalerite and siderite

Castell mine, West Wales. The bulk of the specimen is massive siderite with just the odd observable crystal. The sphalerite is randomly distributed in small pockets, and in a few places the very deep red colour of sphalerite can be seen.