Serpentinite is a metamorphic rock which contains one or more of the serpentine group minerals. This serpentine specimen is from Kynance cove, the Lizard, Cornwall, where there is a very large deposit of serpentinte rock. Serpentine from the general area is usually referred to as "Cornish serpentine"
Microdiorite is a medium grained intrusive igneous rock.This specimen was collected from a coastal super quarry at Pen Clegyr to the west of Porthgain in Pembrokeshire. The feldspars in the microdiorite were altered by hydrothermal activity to clays, making it a very useful material in the production of Porthgain bricks. The coastal location was advantageous in transporting the bricks by sea.
Manganese ore can be quite complex in composition, however the known location and origin of these specimens is helpful. The specimens were collected from Hozatel mine, Port Elizabeth, South Africa. Ore from this mine is described as Hozatel super grade, which has a manganese content of approximately 38% together with finely intergrown braunite and manganese carbonate. The pinkish areas in the specimens are the carbonate, and one specimen has an obvious iron content. Manganese ore was mined here till the 1980s when zones of ferruginization made it become economically unviable. The specimens were collected by a club member at about this time.
The pyrope in this specimen, Mg3Al2(SiO4)3 is a member of the garnet group with high Mg content, and occurs in high grade metamorphic rocks. The dark bands of mineral appear to be pyroxene, which is a Mg,Fe, silicate (Mg,Fe)SiO3 sometimes referred to as hypersthene. The white or light coloured mineral is quartz. The specimen is a gift from Bill Fitches who collected it from the Shai hills, in Ghana.
Penarth beach is a geologists paradise. There is so much variety of structures, and specimens to be found. The Blue Lias rocks of the Jurassic provide a wealth of fossils, some of which I have collected. The older rocks of the Triassic have provided me with lovely specimens of gypsum, including "Satin spar" var. This particular rock specimen lying on the beach caught my attention. It is from the younger Lilstock formation of the Triassic, and is simply a very crisp wave ripple. The pale mudstone was probably laid down in a shallow sea or lagoon. The ripples are obvious, but the small sharply defined deposits on the surface are a mystery.