Dualite, Na30(Ca,Na,Ce,Sr)12(Na,Mn,Fe,Ti)6Zr3Ti3MnSi51O144(OH,H2O,Cl)9, a new zircono-titanosilicate with a modular eudialyte-like structure from the Lovozero alkaline Pluton, Kola Peninsula, Russia |
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| Authors: | A P Khomyakov G N Nechelyustov and R K Rastsvetaeva |
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| Institution: | (1) Institute of Mineralogy, Geochemistry, and Crystal Chemistry of Rare Elements, ul. Veresaeva 15, Moscow, 121357, Russia;(2) All-Russia Institute of Mineral Resources, Staromonetnyi per. 31, Moscow, 119017, Russia;(3) Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 117333, Russia |
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| Abstract: | Dualite has been found at Mount Alluaiv, the Lovozero Pluton, the Kola Peninsula in peralkaline pegmatoid as sporadic, irregularly
shaped grains up to 0.3–0.5 mm across. K-Na feldspar, nepheline, sodalite, cancrinite, aegirine, alkaline amphibole, eudialyte,
lovozerite, lomonosovite, vuonnemite, lamprophyllite, sphalerite, and villiaumite are associated minerals. Dualite is yellow,
transparent or translucent, with conchoidal fracture. The new mineral is brittle, with vitreous luster and white streaks.
The Mohs hardness is 5. The measured density is 2.84(3) g/cm3 (volumetric method); the calculated density is 2.814 g/cm3. Dualite dissolves and gelates in acid at room temperature. It is nonfluorescent. The new mineral is optically uniaxial and
positive; ω = 1.610(1), ɛ = 1.613(1). Dualite is trigonal, space group R3m. The unit cell dimensions are a = 14.153(9), c = 60.72(5) ?, V = 10533(22) ?, Z = 3. The strongest reflections in the X-ray powder pattern d, ? (I,%)(hkl)] are as follows: 7.11(40)(110), 4.31(50)(0.2.10), 2.964(100)(1.3.10), 2.839(90)(048), 2.159(60)(2.4.10, 0.4.20), 1.770(60)(2.4.22,
4.0.28, 440), 1362(50)(5.5.12, 3.0.42). The chemical composition (electron microprobe, H2O calculated from X-ray diffraction data) is as follows, wt %: 17.74 Na2O, 0.08 K2O, 8.03 CaO, 1.37 SrO, 0.29 BaO, 2.58 MnO, 1.04 FeO, 0.79 La2O3, 1.84 C2O3, 0.88 Nd2O3, 0.20 Al2O3, 51.26 SiO2, 4.40 TiO2, 5.39 ZrO2, 1.94 Nb2O5, 0.58 Cl, 1.39 H2O,-O = 0.13 Cl2; they total is 99.67. The empirical formula calculated on the basis of 106 cations as determined by crystal structure is
(Na29.79Ba0.1K0.10)Σ30(Ca8.55Na1.39REE1.27Sr0.79)Σ12 · (Na3.01Mn1.35Fe0.872+Ti0.77)Σ6(Zr2.61Nb0.39)Σ3 (Ti2.52Nb0.48)Σ3(Mn0.82Si0.18)Σ1(Si50.77Al0.23)Σ51 O144(OH)6.54(H2O)1.34·Cl0.98]Σ8.86). The simplified formula is Na30(Ca,Na,Ce,Sr)12(Na,Mn,Fe,Ti)6Zr3Ti3 MnSi51O144 (OH,H2O,Cl)9). The name dualite is derived from Latin dualis (dual) alluding to the dual taxonomic membership of this mineral, which is at the same time zirconosilicate and titanosilicate.
The crystal structure is characterized by two module types (alluivite-like and eudialyte-like) alternating along a threefold
axis with a doubled c period relative to eudialyte and close chemical affinity to rastsvetaevite (Khomyakov et al., 2006a) and labyrynthite (Khomyakov
et al., 2006b). According to the authors’ crystal chemical taxonomy of the eudialyte group, the new mineral belongs to one
of three subgroups characterized by a 24-layered structural framework. Dualite is a mineral formed during the final stages
of peralkaline pegmatite formation. The type material of dualite is deposited at the Fersman Mineralogical Museum, Russian
Academy of Sciences, Moscow.
Original Russian Text ? A.P. Khomyakov, G.N. Nechelyustov, R.K. Rastsvetaeva, 2007, published in Zapiski Rossiiskogo Mineralogicheskogo
Obshchestva, 2007, Pt CXXXVI, No. 4, pp. 68–73.
Approved by the Commission on New Minerals and Mineral Names, International Mineralogical Association, July 8, 2005. |
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