![HS [106 kb]](swe75a.jpg)
![PTS [58 kb]](swe75b.jpg)
Above left: hand specimens of three Cornish rocks, with (left-right) an estimated 7, 26 and 92 volume percent tourmaline. The Botallack sample is the black rock at right. The others are (left) quartz- tourmaline- topaz rock from St. Mewan Beacon and (centre) quartz-tourmaline rock from Roche Rock, respectively associated with the southern and northern margins of the St. Austell granite mass.
Above right: View of two 46x27-mm polished thin sections of the tourmalinite. Some pale blue outer zones of individual crystals can be seen at top right.
Below: Concentric zoning of tourmaline prism, seen in plane-polarized transmitted light. Nominal magnification 50X, long-axis field of view 1.7 mm.
![PMic [266 kb]](swe75c.jpg)
"Rock of the Month # 120, posted June 2011" ---
The counties of Cornwall and Devon are justly famous for their mineral wealth (Embrey and Synes, 1987), and the rugged Cornish cliffs and purple moors are dotted with hundred of abandoned mines (Bancroft and Weller, 1993).
Tourmaline, a family of cyclosilicate (ring-silicate) minerals, is characteristic of many of the district's granitic rocks and of the contact zones against older host rocks. Tourmalines are borosilicates, widespread boron-rich minerals, and many species of tourmaline form elegant gemstones of many colours.
The featured sample is a tourmalinite, largely composed of tourmaline plus minor quartz and accessory phases (plagioclase, siderite, sphene, apatite, ilmenite, hematite). The tourmaline is strongly zoned, and of composition intermediate between schorl and dravite, two iron- and magnesium-rich tourmaline end-member molecules (Wilson 1980, 1996, 2011; Wilson and Long, 1983).
This unusual rock, largely black lustrous tourmaline, "cockle" in Cornish miners' parlance, is from the "tin floor" of Grylls Bunny, on the cliff-side Botallack mine near St. Just, in a spectacular setting overlooking the Atlantic ocean.
References
Bancroft,P and Weller,S (1993) Cornwall's famous mines. Mineral.Record 24, 259-283.
Embrey,PG and Symes,RF (1987) Minerals of Cornwall and Devon. British Museum (Natural History) / Mineralogical Record Inc., 154pp.
Wilson,GC (1980) Ion Microprobe Techniques, with Applications to Analysis of Lithium in Cornish Granites. PhD Thesis, University of Cambridge, 245pp.
Wilson,GC (1996) Tourmaline, intrusion-related. In `Atlas of Alteration: a Field and Petrographic Guide to Hydrothermal Alteration Minerals' (Thompson,AJB and Thompson,JFH editors), Geol.Assoc.Canada Mineral Deposits Division, 119pp., 102-103.
Wilson,GC (2011) Mineralogy of evolved, tourmaline-bearing granitoid rocks from Cornwall. Turnstone Geological Services Limited Report 2010-08P, iv+16pp.
Wilson,GC and Long,JVP (1983) The distribution of lithium in some Cornish minerals: ion microprobe measurements. Mineral.Mag. 47, 191-199.
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