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Barstowite, 3PbCl2.PbCO3.H2O, a new mineral from Bounds Cliff, St Endellion, Cornwall

Published online by Cambridge University Press:  05 July 2018

C. J. Stanley ,
G. C. Jones ,
A. D. Hart ,
P. Keller  and
D. Lloyd
C. J. Stanley
Affiliation:
Department of Mineralogy, British Museum (Natural History), Cromwell Road, London SW7 5BD, UK
G. C. Jones
Affiliation:
Department of Mineralogy, British Museum (Natural History), Cromwell Road, London SW7 5BD, UK
A. D. Hart
Affiliation:
Department of Mineralogy, British Museum (Natural History), Cromwell Road, London SW7 5BD, UK
P. Keller
Affiliation:
Institut fur Mineralogie und Kristallchemie, Universitat Stuttgart, Pfaffenwaldring 55, D-7000 Stuttgart, Germany
D. Lloyd
Affiliation:
18 St Lawrence Lane, Ashburton, Devon, UK
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Abstract

Barstowite, ideally 3PbCl2.PbCO3.H2O is a new mineral from a lead-antimony vein at the south-western end of Bounds Cliff, St Endellion, Cornwall. It occurs as very small (0.2–0.5 mm) subparallel elongate intergrown crystals which form aggregates (up to 3 mm) containing minor inclusions of galena and jamesonite. Other associated minerals are quartz, dolomite, phosgenite, cerussite, sphalerite, pyrite and chalcopyrite. Barstowite is colourless to white and is transparent with an adamantine lustre and white streak. It is brittle, has an imperfect prismatic cleavage, and an uneven fracture. Low reflecting, barstowite has a moderate bireflectance from grey to dark grey in hue and weak to moderate anisotropy. Internal reflections (colourless) are abundant. Measured reflectance values in air and oil are tabulated. Colour values relative to the CIE illuminant C for R1 and R2 in air respectively are: Y% 12.9, 13.3; λd 477, 476; Pe% 3.5, 3.7. VHN100 111 (range 108–117); calculated Mohs hardness is 3. Infrared spectra of barstowite, phosgenite and cerussite are compared.

X-ray studies show that barstowite is monoclinic with space group P21/m and a 4.218(2), b 9.180(2), c 16.673(4) Å, β 91.49(3)°. It has a cell volume of 645.38 Å3 with Z = 2. Dcalc. is 5.76 g/cm3, Dmeas. is 5.71 g/cm3. The strongest six lines of the X-ray powder pattern are [d in Å (I) (hkl)] 4.02 (10) (022); 2.296(8) (040; ; 106); 2.377(6) (007; 026); 4.16(5) (004); 2.108(4) (200); 3.79(3) (014). The name is for the late Richard William Barstow, the Cornish mineral dealer.

Keywords

barstowitenew mineralleadSt EndellionCornwall
Type
Research Article
Information
Mineralogical Magazine , Volume 55 , Issue 378 , March 1991 , pp. 121 - 125
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1991

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