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Sazhinite-(La), Na3LaSi6O15(H2O)2, a new mineral from the Aris phonolite, Namibia: Description and crystal structure

Published online by Cambridge University Press:  05 July 2018

F. Cámara*
Affiliation:
CNR-Istituto di Geoscienze e Georisorse, Sede di Pavia, via Ferrata 1, I-27100 Pavia, Italy
L. Ottolini
Affiliation:
CNR-Istituto di Geoscienze e Georisorse, Sede di Pavia, via Ferrata 1, I-27100 Pavia, Italy
B. Devouard
Affiliation:
Département des Sciences de la Terre, Université Blaise Pascal, UMR 6524 CNRS-UBP-OPGC "Laboratoire Magmas et Volcans", 5 rue Kessler, F-63038 Clermont-Ferrand, France
L. A. J. Garvie
Affiliation:
Department of Geological Sciences, Arizona State University, Tempe AZ 85287-1404, USA
F. C. Hawthorne
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

Abstract

Sazhinite-(La) is a new mineral from the Aris phonolite, Windhoek, Namibia. It occurs in vesicles within the phonolite, together with other species crystallized from late-stage hydrothermal fluids: natrolite, aegirine, microcline, apophyllite, sphalerite, analcime, fluorite, villiaumite, hydroxylapatite, galena, makatite, quartz, eudialyte, kanemite, tuperssuatsiaite and korobitsynite. Sazhinite-(La) forms small euhedral crystals up to 1 mm long and 0.4 mm wide, elongated along [001] and flattened on (010), exhibiting the forms {h0l}, {100} and {001}. It has good cleavage parallel to {010} and {001}. Twinning was not observed. Crystals are brittle with a Mohs hardness of 3, creamy white with a white streak, vitreous to pearly lustre, and translucent to transparent. In plane-polarized light, crystals are colourless with a = Z, b = Y, c = X. It is biaxial positive with α = 1.524, β = 1.528, γ = 1.544, all ±0.002, 2Vz(obs) = 46(1)°, and 2Vz(calc.) = 53.6°.

Sazhmite-(La) is orthorhombic Pmm2, a = 7.415(2), b = 15.515(3), c = 7.164(1) Å, and V = 824.2 Å3. One crystal was studied by X-ray diffraction, electron microprobe and secondary ion mass spectrometry (SIMS) microanalysis, leading to the average composition (Na2.87K0.02Sr0.01)Σ2.90 [La0.41Ce0.35Pr0.02Nd0.04(Sm,Gd,Dy,Er,Yb)Σ0.01Th0.09U0.01Y0.01Zr0.01Ca0.08Li0.01]Σ1.04 (S15.87S0.06B0.01) (O14.86F0.14).(H2O)2.

Weighted full-matrix least-squares refinement on 3369 reflections yielded R all = 3.8%. The structure is built of corrugated [Si6O15]6- layers linked by [7]-coordinated REE and R4+ cations. This framework leaves channels that contain three [5]- and [6]-coordinated Na cations per formula unit that compensate for the residual charge on the silicate layers. The SIMS analyses confirm a Na content of 3 atoms per formula unit, leading to an ideal formula of Na3LaSi6O15(H2O)2. The third Na atom is bonded to H2O groups and therefore the total content of both Na and H2O may be reduced to 2 and 1 per formula, respectively. The depletion in Na allows for the entrance of high-charge cations such as Th4+.

Type
Editorial
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2006

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Supplementary material: File

Cámara et al. supplementary material

Table 6. Observed and calculated structure factors for sazhinite-(La) Vol. 70, June 2006

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