Original paper

Crystal chemistry of cation-exchanged forms of epistolite-group minerals. Part II. Vigrishinite and Zn-exchanged murmanite

Lykova, Inna S.; Pekov, Igor V.; Zubkova, Natalia V.; Yapaskurt, Vasiliy O.; Chervonnaya, Nadezhda A.; Zolotarev, Andrey A.; Giester, Gerald

European Journal of Mineralogy Volume 27 Number 5 (2015), p. 669 - 682

published: Oct 1, 2015

DOI: 10.1127/ejm/2015/0027-2469

BibTeX file

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The crystal structure of vigrishinite, an epistolite-group heterophyllosilicate with essential Zn, has been reinvestigated; the ideal end-member formula is revised to Zn2Ti4-x (Si2O7)2O2(OH,F,O)2(H2O,OH,〈)4 with x<1. Structure models of Zn-exchanged forms of murmanite after 5 and 24 hour experiments with 1N ZnSO4 solution at 90 °C have been obtained from single-crystal X-ray diffraction data. The structural formulae are [B1]Ca0.04{Na1.22(Ti1.19Mn0.60Nb0.21)}{[A1,A2]Zn1.03(Ti1.64Nb0.36)[Si2O7]2}O2(O,OH)2(H2O)4 for vigrishinite and {Na1.14(Ti1.45Mn0.50Nb0.05)}{[A1]Ca0.77 [A2]Zn0.13(Ti1.85Nb0.15)[Si2O7]2}O2(OH,O)2(H2O)4 and [B1]Zn0.15 [B2]Ca0.18{Na1.06(Ti1.32Mn0.60Nb0.08)}{[A1]Zn0.70 [A2]Ca0.12(Ti1.74Nb0.26)[Si2O7]2}O2(OH,O)2(H2O)4 for the 5 and 24 hour Zn-exchanged forms of murmanite, respectively (braces give successively the contents of the octahedral O and heteropolyhedral H sheets). The triclinic (P–1) unit-cell parameters are respectively: a = 8.7127(17), 8.871(3), 8.748(2) Å; b = 8.6823(17), 8.844(6), 8.724(2) Å; c = 11.746(2), 11.734(6), 11.675(3) Å; α = 91.481(4), 92.75(3), 92.503(13)°; β = 98.471(4), 97.60(4), 97.846(13)°; γ = 105.474(4), 106.23(2), 105.875(13)°; V = 845.0(3), 872.7(8), 845.9(4) Å3. Our data (1) confirm that vigrishinite was formed as a result of natural ion-exchange of murmanite Na4Ti4 (Si2O7)2O4 · 4H2O with Zn2+ in low-temperature solutions; (2) prove that direct transformation of lomonosovite Na4Ti4(Si2O7)2O4 · 2Na3PO4 into vigrishinite, without prior leaching of Na+ and PO4 3– from the former and formation of murmanite, is unlikely; (3) suggest the following ion-exchange mechanism: during the early stage Na+ leaches into the solution whereas Ca2+, a common admixture in murmanite, migrates into one of the sites in the H sheet, leaving another site vacant for further entry of Zn; in the next stage Zn2+ enters the emptied site in the H sheet and, in small amount, into the interlayer whereas Ca2+ partly moves from the H sheet into the interlayer; (4) show the transformation of the murmanite-type unit cell (P1; V ≈ 440 Å3) into the vigrishinite-type cell with a and b parameters corresponding to the ab face diagonals of the former during the first stage of the exchange with Zn, due to ordering in the H sheet. New findings of vigrishinite in two pegmatites of the Lovozero massif, Kola peninsula, Russia (at Severnyi open pit, Mt. Alluaiv and at Pegmatite #60, Mt. Karnasurt) in the same setting as at the type locality, i.e. only in close contact with cavities after dissolved sphalerite, show that the ion exchange in epistolite-group heterophyllosilicates is not uncommon in Nature. Our data indicate there is a continuous solid solution between murmanite and vigrishinite.


zvyaginitevigrishiniteheterophyllosilicate minerallomonosoviteion exchangelovozero alkaline massiftitanosilicatemurmaniteperalkaline rockcrystal structure