Wayneburnhamite, Pb9Ca6(Si2O7)3(SiO4)3,
an apatite polysome
– the Mn-free analogue of ganomalite from
Crestmore, California
Anthony R. Kampf
Mineral Sciences Department, Natural History Museum of Los Angeles
County,
900 Exposition Boulevard, Los Angeles, CA 90007, USA
Robert M Housley, George R. Rossman
Division of Geological and Planetary Sciences, California Institute of
Technology,
Pasadena, CA 91125, USA
Abstract
Wayneburnhamite (IMA2015-124), Pb9Ca6(Si2O7)3(SiO4)3,
is a new mineral from the Commercial quarry, Crestmore, Riverside
County, California, where it occurs as a metasomatic mineral on
fracture surfaces in vesuvianite/wollastonite rock. Wayneburnhamite
crystals are sky blue hexagonal tablets and prisms up to 0.5 mm in
maximum dimension. The streak is white. Crystals are transparent to
translucent with vitreous to resinous luster. The Mohs hardness is 3½,
the tenacity is brittle, the fracture is conchoidal, and there is no
cleavage. The calculated density is 5.271 g/cm3.
The mineral is optically uniaxial (+), with ω = 1.855(5) and ε =
1.875(5) (white light). The pleochroism is E sky blue and O lighter sky
blue; E > O weak. Raman and infrared spectra are consistent with
the crystal structure, but suggest a very minor hydrous component. The
empirical formula (based on 9 Si apfu) is (Pb8.33Sr0.04□0.63)Σ9.00(Ca5.40Cu2+0.27□0.33)Σ6.00Si9S0.21O32.64Cl0.05.
Wayneburnhamite is hexagonal, P-6, a = 9.8953(9), c = 10.2054(7) Å, V =
865.40(17) Å3, and Z = 1. The eight strongest
lines in the X-ray powder diffraction pattern are [dobs in Å(I)(hkl)]:
4.95(52)(110); 4.45(64)(111); 3.550(77)(112); 3.232(54)(120);
3.086(100)(121); 2.847(60)(300); 2.798(48)(113); and 2.734(83)(212).
The structure determination (R1 = 3.01% for 1063 Fo > 4σF) shows
wayneburnhamite to be an apatite polysome isostructural with
ganomalite, differing only in that the site occupied dominantly by Mn
in the structure of ganomalite is occupied dominantly by Ca in the
structure of wayneburnhamite. The structure refinement of
wayneburnhamite appears to represent a rare case in which the
approximate locations of the Pb2+ 6s2
lone-electron pairs can be seen as electron density residuals.