Canadian Mineralogist 

About: Canadian Mineralogist is an academic journal published by Mineralogical Association of Canada. The journal publishes majorly in the area(s): Pegmatite & Crystal structure. It has an ISSN identifier of 0008-4476. Over the lifetime, 4517 publications have been published receiving 108924 citations.

The crystal chemistry of hexavalent uranium; polyhedron geometries, bond-valence parameters, and polymerization of polyhedra

Abstract: The geomeffy, bond valences, and polymerization ofhexavalent uranium polyhedra from 105 well-refined structures are analyzed. The Utu cation is almost always present in crystal stnrctures as part of a nearly linear (UOr)z* uranyl ion that is coordinated by four, five or six equatorial anions in an approximately planar arangement perpendicular to the uranyl ion, giving square, pentagonal and hexagonal bipyramids, respectively. The Utu-O7\" bond length (Oy,: uranyl-ion O atom) is independent of the equatorial anions of the polyhedra;-averages of all polyhedra tlat contain uranyl ions ffs; I6lIJ6f-Or. = 1.79(3), mg0.-.9 a,= 1.79(4), and t8lu6+-Our = 1.78(3) A. Not a[ r6lu6+ polyhedra contain uranyl ions; there is a continuous series of coordiaation polyhedr4 from square bipyramidal polyhedra with uranyl ions to holosymmehic octahedral geometry. The mUo* and t8lu6+ polyhedra invariably contaitl a uranyl ion. The equatorial U6.-0 (0: O,-, OH-) bond-lengths of uranyl polyhedra depend upon coordhation number; averages for all polyhedra are t6lu6+-dq = 2,28(5), rlUot-$* = 2.37(9), afi t8tlJ6+-$q2.47 (12) A. Cunently available bond-valence parameters for U& are unsatisfactory for determining bond-valence sums. Coordination-specific bond-valence paxameters have been derived for U6|, together with parameters applicable to all coordination geometries. The parameters give bond-valence sums for Ue of -6 vlr and reasonable bond-valences for Uc,-Ou, bonds. The bond-valence paraneters facilitate the recognition of Ua, U5+ and U6| catiotrs in refined crystal structures. The crystal-chemical consfraints ofpolyhedral polymerization in uranyl phases are discussed.

The classification of granitic pegmatites revisited

Abstract: The classification of granitic pegmatites was frequently attempted during the past century, with variable degrees of success and applicability. Internal structure, paragenetic relationships, bulk chemical composition, petrogenetic aspects, nature of parent medium, and geochemical features were applied. However, all schemes were marked by contemporary degrees of understanding of these parameters, and most attempts were hindered by ignoring differences in geological environment. Substantial progress was achieved only since the late 1970s. The classification is approached here from two directions, based on but broadened and refined from earlier works by Ginsburg and Cerný. The first concept deals with geological location, leading to division of granitic pegmatites into five classes (abyssal, muscovite, muscovite – rare-element, rare-element, and miarolitic), most of which are subdivided into subclasses with fundamentally different geochemical (and in part geological) characteristics. Further subdivision of most subclasses into types and subtypes follows more subtle differences in geochemical signatures or P–T conditions of solidification, expressed in variable assemblages of accessory minerals. The second approach is petrogenetic, developed for pegmatites derived by igneous differentiation from plutonic parents. Three families are distinguished: an NYF family with progressive accumulation of Nb, Y and F (besides Be, REE, Sc, Ti, Zr, Th and U), fractionated from subaluminous to metaluminous A- and I-type granites that can be generated by a variety of processes involving depleted crust or mantle contributions; a peraluminous LCT family marked by prominent accumulation of Li, Cs and Ta (besides Rb, Be, Sn, B, P and F), derived mainly from S-type granites, less commonly from I-type granites, and a mixed NYF + LCT family of diverse origins, such as contamination of NYF plutons by digestion of undepleted supracrustal rocks.