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Barrydawsonite-(Y), Na1.5CaY0.5Si3O9H: a new pyroxenoid of the pectolite–serandite group

Published online by Cambridge University Press:  02 January 2018

R. H. Mitchell ,
M. D. Welch ,
A. R. Kampf ,
A. K. Chakhmouradian  and
J. Spratt
R. H. Mitchell*
Affiliation:
Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada P7B 5E1
M. D. Welch
Affiliation:
Department of Earth Sciences, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
A. R. Kampf
Affiliation:
Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA
A. K. Chakhmouradian
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
J. Spratt
Affiliation:
Department of Earth Sciences, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
*
*E-mail: rmitchel@lakeheadu.ca
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Abstract

The new pyroxenoid barrydawsonite-(Y) occurs at the Merlot Claim, North Red Wine Pluton, Labrador, Canada (62°32'38.54'' W; 54°08'1.37'' N). The host rock is a metamorphosed eudialyte syenite consisting of eudialyte, potassic arfvedsonite, jadeitic aegirine, nepheline, albite and potassium feldspar with accessory Y-bearing pectolite, britholite and steenstrupine. Barrydawsonite-(Y) crystals commonly have discrete thin rims of Y-bearing pectolite. The average empirical formula (based on nine anions p.f.u.) is Na1.54Ca0.74Mn0.15Fe0.07Y0.38Nd0.01Sm0.01Gd0.02Tb0.01Dy0.04Ho0.01Er0.02Yb0.01Si3.00O9H. The simplified formula is Na1.5Y0.5CaSi3O9H. Barrydawsonite-(Y) is related to pectolite by the substitution ½[NaM3+Ca–2] (M3+ = Y,REE), and is exceptional in being the only member of the pectolite group that has the structure of the monoclinic M2abc polytype. The crystal structure has been determined in monoclinic space group P21/a: a = 15.5026(2), b = 7.0233(1), c = 6.9769(1) Å, β = 95.149(1)°, V = 756.58(2) Å3(Z = 4). Final agreement indices are R1 = 0.038, wR2 = 0.068, Goof = 1.136. The asymmetric unit of barrydawsonite-(Y) has three metal sites: M(1) = Ca, M(2) = Na0.5(Y,REE)0.5, M(3) = Na. M(1) and M(2) are octahedrally-coordinated sites, whereas M(3) is [8]-coordinated as in pectolite and serandite. The structural formula for the empirical composition is M(3)Na1.00M(2)(Na0.50Y0.38REE0.13)Σ=1.01M(1)(Na0.04Ca0.74Mn0.152+Fe0.072+)Σ=1.00Si3O9H. There is excellent agreement between the refined site-scattering values and those calculated based upon the structural formula.

Barrydawsonite-(Y) is biaxial (+) with α = 1.612(1), β = 1.617(1), γ = 1.630(1) (white light) and 2V = 63(1)°. The five strongest peaks in the X-ray powder diffraction pattern are [dobs (Å), Iobs%, (hkl)]: [2.905, 100, (023)], [3.094, 30, (210,211,121,202)], [1.7613, 29, (127,323,040)], [3.272, 27, (202,104)], [1.7016, 27, (140,227,325)].

Keywords

barrydawsonite-(Y)pyroxenoidpectolite–seranditepolytype
Type
Research Article
Information
Mineralogical Magazine , Volume 79 , Issue 3 , June 2015 , pp. 671 - 686
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2015

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