A NEW MINERAL, CHRISSTANLEYITE, AG2PD3SE4, FROM HOPES NOSE, TORQUAY, DEVON, ENGLAND

Chrisstanleyite, Ag2Pd3Se4, is a new mineral from gold-bearing carbona te veins in Middle Devonian limestones at Hope's Nose, Torquay, Devon, England. It is associated with palladian and argentian gold, fischess erite, clausthalite, eucairite, tiemannite, umangite, a Pd arsenide-an timonide (possibly mertieite II), cerussite, calcite and bromian chlor argyrite. Also present in the assemblage is a phase similar to oosterb oschite, and two unknown minerals with the compositions, PdSe2 and HgP d2Se3. Chrisstanleyite occurs as composite grains of anhedral crystals ranging from a few pm to several hundred mu m in size. It is opaque, has a metallic lustre and a black streak, VHN100 ranges from 371-421, mean 395 kp/mm(2) (15 indentations), roughly approximating to a Mohs h ardness of 5. D-calc = 8.308 g/cm(3) for the ideal formula with Z = 2. In plane-polarised reflected light, the mineral is very slightly pleo chroic from very light buff to slightly grey-green buff; is weakly bir eflectant and has no internal reflections. Bireflectance is weak to mo derate (higher in oil). Anisotropy is moderate and rotation tints vary from rose-brown to grey-green to pare bluish grey to dark steel-blue. Polysynthetic twinning is characteristic of the mineral. Reflectance spectra and colour values are tabulated. Very little variation was not ed in eleven electron-microprobe analyses on five grains, the mean is: Ag 25.3, Cu 0.17, Pd 37.5, Se 36.4, total 99.37 wt.%. The empirical f ormula (on the basis of Sigma M + Se = 9) is (Ag2.01Cu0.02)(Sigma 2.03 ) Pd3.02Se3.95, ideally Ag2Pd3Se4 Chrisstanleyite is monoclinic, a 6.3 50(6). b 10.387(4), c 5.683(3) Angstrom, beta 114.90(5)degrees, space group P2(1)/m (11) or P2(1)(4). The five strangest X-ray powder-diffra ction lines [d in Angstrom (1)(hkl)] are: 2.742 (100) (-121), 2.688 (8 0) (-221), 2.367 (50) (140), 1.956 (100) (-321,150) and 1.829 (30) (-3 21,042). The name is in honour of Dr Chris J. Stanley of The Natural H istory Museum in London. The mineral and its name have been approved b y the Commission on New Minerals and Mineral Names of the Internationa l Mineralogical Association.