Welshite, Ca2Mg4Fe3+Sb5+O2[Si4Be2O18], a New Member of the Aenigmatite Group

Paul B. Moore
Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois 60637, U.S.A.

Summary: Welshite, triclinic, Ca2Mg4Fe3+Sb5+O2 [Si4Be2O18], a = 10·28 Å, b = 10·69 Å, c = 8·83 Å, α = 106·1°, β = 96·3°, γ = 124·8°, P¯1 with pseudo-monoclinic cell am = 9·68(1) Å, bm = 14·77(2) Å, cm = 5·14(1) Å, βm = 101·5(2)°, c-centred, occurs as lustrous subadaman-tine, thick prismatic reddish-black crystals up to 3 mm in greatest dimension associated with roméite, adelite, swedenborgite, berzeliite, manganoan phlogopite, richterite in crystalline dolomite from Làngban, Sweden. Forms include {itb}{010}, a{100}, k{130}, m{110}, j{021}, p{111}, q{121}, v{131}, r{¯111}, s{¯121}, t{¯131}, and v{¯141} (monoclinic setting). Streak pale-brown, hardness 6, specific gravity 3·77, fracture conchoidal, no cleavage, α = 1·81(1), γ = 1·83(1), 2E(obs.) ≈45°, no noticcable pleochroism.

Welshite is a new member of the aenigmatite-rhönite group and obtains from the substitution Sb5+→ Ti4+ and Be2+ → Al3+ in rhönite.

Mineralogical Magazine; March 1978 v. 42; no. 321; p. 129-132; DOI: 10.1180/minmag.1978.042.321.19
© 1978, The Mineralogical Society
Mineralogical Society (www.minersoc.org)