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

Abstract

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 ¼ 8.7127(17), 8.871(3), 8.748(2) A ° ; b ¼ 8.6823(17), 8.844(6), 8.724(2) A ° ; c ¼ 11.746(2), 11.734(6), 11.675(3) A ° ; a ¼ 91.481(4), 92.75(3), 92.503(13) ; b ¼ 98.471(4), 97.60(4), 97.846(13) ; g ¼ 105.474(4), 106.23(2), 105.875(13) ; V ¼ 845.0(3), 872.7(8), 845.9(4)A ° 3. Our data (1) confirm that vigrishinite was formed as a result of natural ion-exchange of murmanite Na4Ti4(Si2O7)2O4 4H2O with Zn2þ in low-temperature solutions; (2) prove that direct transformation of lomonosovite Na4Ti4(Si2O7)2O4 2Na3PO4 into vigrishinite, without prior leaching of Naþ and PO4 3– from the former and formation of murmanite, is unlikely; (3) suggest the following ion-exchange mechanism: during the early stage Naþ leaches into the solution whereas Ca2þ, 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 Zn2þ enters the emptied site in the H sheet and, in small amount, into the interlayer whereas Ca2þ partly moves from the H sheet into the interlayer; (4) show the transformation of the murmanite-type unit cell (P 1; V 440A ° 3) 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.