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

¹ CNR-Istituto di Geoscienze e Georisorse, Sede di Pavia, via Ferrata 1, I-27100 Pavia, Italy
² 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
³ Department of Geological Sciences, Arizona State University, Tempe AZ 85287-1404, USA
⁴ Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

^* E-mail: camara{at}crystal.unipv.it

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 = 1.524, ß = 1.528, = 1.544, all ±0.002, 2V_Z(obs) = 46(1)°, and 2V_Z(calc.) = 53.6°.

Sazhinite-(La) is orthorhombic Pmm2, a = 7.415(2), b = 15.515(3), c = 7.164(1) Å, and V = 824.2 ų. One crystal was studied by X-ray diffraction, electron microprobe and secondary ion mass spectrometry (SIMS) microanalysis, leading to the average composition (Na_2.87K_0.02Sr_0.01)_2.90 [La_0.41Ce_0.35Pr_0.02Nd_0.04(Sm,Gd,Dy,Er,Yb)_0.01Th_0.09U_0.01Y_0.01Zr_0.01Ca_0.08Li_0.01]_1.04^T(Si_5.87S_0.06B_0.01) (O_14.86F_0.14)·(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⁺.

KEYWORDS: sazhinite-(La), new mineral, EMPA, SIMS, SCXRD, rare-earth minerals