The crystal structure of mineral sakhaite Ca
48[Mg
13.2(Fe, Mn)
2.8](CO
3)
16{ Al[SiO
3.75(OH)
0.25]
4}(BO
3)
28[(H
2O)
3.3(HCl)
3.3] from the Solongo deposit, Zabaykalye, is established (
R 0.047) by X-ray diffraction analysis (Bruker Smart CCD diffractometer, 2θ: θ scan, λMo
K
α radiation, graphite monochromator):
a = 14.679(2) Å, sp. gr.
Fd3
m,
Z = 1, ρ
calcd = 2.99 g/cm
3. It is shown that some part of BO
3 triangles in the structure of sakhaite from Solongo is replaced with five-member complexes {Al[SiO
3(O,OH)]
4}, established previously in the harkerite structure. Three schemes of isomorphism, which are characteristic of the sakhaite-harkerite
mineral series, are selected and the general formula is proposed: Ca
48(Mg,Fe,Mn)
16(CO
3)
16{Al[SiO
3(O, OH)]
4}
y(B
3)
32−y
·
n(H
2O,HCl) (
Z = 1),
y
max = 8,
n
max = 16 −
y. The structural relationship of sakhaite-harkerite minerals with borate-sulfates of the tychite family Na
6(Mg,Fe,Mn)
2(BO
3)
4(SO
4) is revealed. The correlation between the number of harkerite fragments forming the mineral structure, the structural symmetry,
and the degree of imperfection is ascertained.
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Translated from Kristallografiya, Vol. 50, No. 2, 2005, pp. 226–234.
Original Russian Text Copyright © 2005 by Yakubovich, Still, Gavrilenko, Urusov.