A striking coarse granite from the south coast of Cornwall. This sample is oriented with its top upwards: a flat top, above which was a roof of grey slate. The piece samples the very top of a sheet of granite melt which, upon crystallizing, recorded the exotic chemistry of its volatile-rich "mother liquor" in stone. Coarse pink crystals of orthoclase feldspar grow in a matrix of smaller grains of glassy grey quartz, creamy white plagioclase feldspar, and lesser brown zinnwaldite mica. Thin striated prisms of glossy black tourmaline (the iron-rich variety, schorl) are concentrated within 2.5 cm (one inch) of the roof of the sheet. Sample SWE061, part of an extensive collection used in a petrographic and geochemical study, with emphasis on tourmaline- rich rocks (Wilson, 1980).

"Rock of the Month #100, posted for October 2009" ---

A striking pegmatitic granite with rapid variation in grain size from >50 mm to <1 mm in patches of finely granular (aplitic) texture.

The Cornish granite massif forms the rugged southwestern extremity of England. The Hercynian-age granite masses of Cornwall and Devon have been studied in great detail over the years, both during and after the 19th-century apex of the local mining industry, when tin and copper, plus a host of other mineral products, were won from the local veins. Central to the majority of the mineral deposits is the batholithic granite mass, which rises in a number of stocks to reach surface on the British mainland from Dartmoor west to Land's End (Exley and Stone, 1964). Many of the granitoid rocks are unremarkable biotite granites, but others have more evolved compositions, enriched in lithium, boron, fluorine and other elements (e.g., Stone, 1992).

The sample displayed here is from the south coast of Cornwall, in a series of rocky promontories between the beaches of Porthleven Sands to the east, and Praa Sands to the west (Ordnance Survey, 1974). The cliffs of Tremearne, Megiliggar Rocks, Trewavas Head and Rinsey Head are part of a small granite body, the Tregonning- Godolphin granite, which displays interesting field relations at the roof of the granite, presumably a small stock rising from the larger batholith at depth. Granite sheets at Tremearne show lateral differentiation from leucogranite to aplite and pegmatite (Stone, 1969). This sample is from the top of a granite sheet, the tourmaline prisms tending to crystallize with their long axes normal, or at high angles to the overlying contact with the slate country rock. On closer inspection, many of the tourmaline needles at this locality are hollow, infilled by quartz or white feldspar.

References

Exley,CS and Stone,M (1964) The granitic rocks of South-West England. In `Present Views on Some Aspects of the Geology of Cornwall and Devon' (Hosking,KFG and Shrimpton,GJ editors), Roy.Geol.Soc.Cornwall, 330pp., 131-184.

Ordnance Survey (1974) Land's End and The Lizard. Ordnance Survey 1:50,000 scale first series map sheet 203.

Stone,M (1969) Nature and origin of banding in the granitic sheets, Tremearne, Porthleven, Cornwall. Geol.Mag. 106, 142-158.

Stone,M (1992) The Tregonning granite: petrogenesis of Li-mica granites in the Cornubian batholith. Mineral.Mag. 56, 141-155.

Wilson,GC (1980) Ion Microprobe Techniques, with Applications to Analysis of Lithium in Cornish Granites. PhD Thesis, University of Cambridge, 245pp.