Octahedral Cation Ordering in Li-rich Micas

Emanuele Benincasa (brigatti@unimo.it), Maria Franca Brigatti, Cristina Lugli, Luca Medici & Luciano Poppi

Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia, Via S. Eufemia, 19, I-41100 Modena, Italy

The crystal structure and M site populations of a series of Li-rich di- and tri-octahedral micas (1 M and 2 M1 polytype) were determined by single crystal X-ray diffraction data. Trioctahedral micas fall in the polylithionite-siderophyllite-annite field, being 0 < Li < 2.82, 0.90 < Fe_total < 5.00 and 0.26 < ^[6]Al < 2.23 apfu, whereas dioctahedral micas are members of the trilithionite-muscovite join (Rieder et al., 1999). In trioctahedral micas the results of the X-ray study showed ordering of trivalent cations (mainly Al) in a cis-octahedral site (M2 or M3), which leads to a lowering of the layer symmetry from C12/m(1) (siderophyllite and annite crystals) to C12(1) diperiodic group (lithian siderophyllite and ferroan polylithionite crystals). On the basis of mean bond length, the ordering scheme of octahedral cations is mostly meso-octahedral, whereas the mean electron count at each M site suggests both meso- and hetero-octahedral ordering. As the siderophyllite content increases, so do the a, b, and c unit cell parameters, as well as the refractive indices. The tetrahedral rotation angle, (alpha), is generally small and roughly increases with polylithionite content, whereas the basal oxygen out-of-plane tilting, z, is sensitive both to octahedral composition and degree of order. Dioctahedral Li-rich micas present residual electron density in the position classically attributed to the vacancy, suggesting that M1 site is involved in the Li substitution.

Rieder M, Cavazzini G, D'Yakonov Y, Frank-Kamenetskii VA, Gottardi G, Guggenheim S, Koval PV, Müller G, Neiva AMR, Radoslovich EW, Robert JL, Sassi FP, Takeda H, Weiss Z & Wones DR, The Canadian Mineralogist, 36, 905-912, (1998).

Effect of Intensive Variables on Coexisting Di- and Trioctahedral Micas: A Crystal-Chemical Approach

Emanuele Benincasa (brigatti@unimo.it), Maria Franca Brigatti, Paola Frigieri, Luca Medici & Luciano Poppi

Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia, Via S. Eufemia, 19, I-41100 Modena, Italy

The crystal structure and octahedral site population of coexisting di- and trioctahedral micas from metapelites and peraluminous granites were determined from single-crystal x-ray diffraction data to evaluate the structural variation produced in both micas by intensive variables. Trioctahedral micas are members of phlogopite-annite join and cover the compositional range 0.39 < Mg/(Mg+Fe²⁺) < 0.74 and 0.90 < ^[6] Al < 1.22 apfu, whereas dioctahedral micas, near to the end-member muscovite, are characterised by a variable extent of octahedral substitution [4.14 < (Mg+Fe²⁺+Ti+Mn) < 4.79 apfu].Trioctahedral micas, mostly 1 M polytype, show a consistent preference of Al³⁺ for M2 site thus resulting in a reduction of <M2-O> bond distances as the Al content increases. Fe and Mg are distributed over the M1 and M2 sites with a preference of Fe for M1 in accordance with the intercrystalline partition coefficient between M1 and M2 sites in di-and trioctahedral micas D^M1/M2 [D^M1/M2= (Fe/Mg)_M1/(Fe/Mg)_M2] ranging between 2.00 and 5.0. The refinements of muscovite-2 M₁ crystals indicate the typical dioctahedral order with vacancy in the trans M1-site. Elements heavier than Al occupy M2, a circumstance which agrees with amount of phengitic substitution determined by chemical analysis. The residual electron density at M1 increases with phengitic substitution. The variation of dioctahedral mica cell volume is correlated with the Al partition coefficient between M2 sites of di- and trioctahedral micas [D (Al) _M2 ^Bt/Ms] and with the Al saturation index of the host rock. The behaviour of the Ti component is opposed to that of Al showing an increase of M2 site and unit cell volumes in coexisting micas thus possibly reflecting the influence of temperature during mica growth.

Formation Conditions of Gaspéite and Metastable Ni-rich Minerals from Dubostica, Bosnia and Hercegovina

Vladimir Bermanec (vberman@public.srce.hr)¹, Galiba Sijaric² & Goran Kniewald (kniewald@gs.ucsd.edu)³

¹ Department of Mineralogy and Petrology, Faculty of Sciences, 10000 Zagreb, Croatia

² Faculty of Natural Sciences and Mathematics, University of Sarajevo, 71000 Sarajevo, Bosnia and Hercegovina

³ Department of Marine and Environmental Research, Rudjer Boskovic Institute, 10000 Zagreb, Croatia

A suite of nickel minerals - gaspéite, retgersite and nickelhexahydrite was found in veins crossing altered ultramafic rocks from the Dubostica locality in Bosnia and Hercegovina. Unit cell dimensions of the minerals are: gaspéite a 4.648(3), c 14.95(2) Å (rhombohedral a 5.660 Å, (alpha) 48.49°) V 279.7 ų; retgersite a 6.787(5), c 18.27(2) Å, V 842(1) ų; nickelhexahydrite a 9.925(7), b 7.232(7), c 24.16(2), ß 98.55(7), V 1715(2) ų. The chemical composition of the Dubostica gaspéite, obtained by ICP-AES is: NiO 49.63, MgO 0.17, CaO 0.45, FeO 6.36, CO₂ 43.25, (total 99.87 wt.%). This gives the empirical formula (based on 3 oxygens)(Ni_0.73Fe_0.10Ca_0.01)(sum)0.84C_1.08O_3.00. The analysis of an equilibrium Eh-pH model for ionic and complexed nickel species, in terms of the observed mineral association, indicates that the attainment of a series of kinetically controlled transitory equilibria results in the paragenetic formation of metastable mineral phases.

The Pseudobinary System CdS - CdSe and its Bearings on the "Locked-type" Zircon Based Ceramic Pigments

Gian Piero Bernardini (gpb@steno.geo.unifi.it)¹, Daniele Borrini (dborrini@steno.geo.unifi.it)¹, Costanza Danti (cdanti@steno.geo.unifi.it)², Francesco Di Benedetto (cdanti@steno.geo.unifi.it)² & Giuseppe Mazzetti (musminfi@cesit1.unifi.it)²

¹ Dpt. of Earth Sciences, University of Florence, via La Pira, 4, 50121 Firenze, Italy

² Natural History Museum, Mineralogical Section, University of Florence, via La Pira, 4, 50121 Firenze, Italy

The locked-type zircon based ceramic pigments have recently received much attention mainly for their thermal and colouring properties (thermal stability and expansion, possible blending with other stains, etc.). Among diverse synthetic products the terms of the CdS - CdSe solid solution appear to be very promising ranging in colour from yellow to deep red. Although the "locking" procedure of the chromophore compounds is well established as an industrial process, the relationships between the enclosing zircon and the "guest" phase still await a better understanding. The phase relations in the CdS - CdSe system have been established in detail, therefore, using mainly DTA and "appearance of phase" procedures on synthetic compounds. The locking mechanism has been experimentally investigated starting both from presynthesized compounds [Cd(S,Se) and ZrSiO₄] and from CdCO₃, elemental S and Se, ZrO₂ and SiO₂. These products have been characterised by XRD, XRF, EMPA, SEM and TEM analyses and the results compared with those obtained on a commercial pigment. For the CdS-CdSe system a complete s.s. below 500°C has been confirmed through an almost perfect linear correlation between the cell volume and the composition of several synthetic terms. As concerns the locked-type zircon compound, the syntheses starting from the CdCO₃, S, Se, ZrO₂, SiO₂ and NaF have been performed in gas-tight (up to ~ 4 kBars) inconel vessels. NaF acting as a mineraliser, the incorporating reaction takes place contemporarily to the decomposition of CdCO₃ and the formation of ZrSiO₄. The Cd(S,Se) chromophore locked in the ZrSiO₄ crystals has been revealed only through electron microscopic devices (SEM, TEM, EMPA), before and after washing procedures to remove the non-locked Cd(S,Se) compounds. Preliminary tests of pigment application to ceramic glazes point to a satisfactory chromatic efficacy.