Miguel Angel Galliski

The Cretaceous paleorift in northwestern Argentina: A petrologic approach

Abstract The development of the Cretaceous-early Eocene basin of northwestern Argentina can be divided into three main magmatic phases on the basis of preliminary petrologic data. The oldest phase (130-100 Ma) is divided into an early stage of anorogenic plutonism, with subalkaline, alkaline, and minor peralkaline granitic intrusives, and a volcanic stage in which alkaline rocks characterized by trachytes, basanites and foidites prevail. The second phase (80-75 Ma) is characterized by an alkaline suite with basanites, hawaiites and tephriphonolites. The last phase (65-60 Ma) consists of lamproitic sills and basic lava flows. The two first phases correspond, respectively, to prerifting and initial rifting stages. According to the magmatic, tectonic, and sedimentary features observed, it is suggested that this basin is a foreland paleorift, of low volcanicity type, that developed along the western side of South America from the Early Cretaceous to the Eocene—at which time the rift basin was closed by the Incaic diastrophic phase.

Ferrotitanowodginite, Fe (super 2+) TiTa 2 O 8 , a new mineral of the wodginite group from the San Elias pegmatite, San Luis, Argentina

Simmonsite, Na2LiA1F6, a new mineral of pegmatitic-hydrothermal origin, occurs in a late-stage breccia pipe structure that cuts the Zapot amazonite-topaz-zinnwaldite pegmatite located in the Gillis Range, Mineral Co., Nevada, U.S.A. The mineral is intimately intergrown with cryolite, cryolithionite and trace elpasolite. A secondary assemblage of other alumino-fluoride minerals and a second generation of cryolithionite has formed from the primary assemblage. The mineral is monoclinic, P21 or P21/m, a = 7.5006(6) Å, b = 7.474(1) Å, c = 7.503(1) Å, β = 90.847(9)o, V = 420.6(1) Å, Z = 4. The four strongest diffraction maxima [d (Å), hkl, I/I100] are (4.33, 111 and 111 _ , 100); (1.877, 400 and 004, 90); (2.25, 131 _ , 113, 131 and 311, 70); and (2.65, 220, 202, 022, 60). Simmonsite is pale buff cream with white streak, somewhat greasy, translucent to transparent, Mohs hardness of 2.5–3, no distinct cleavage, subconchoidal fracture, no parting, not extremely brittle, Dm is 3.05(2) g/cm, and Dc is 3.06(1) g/cm. The mineral is biaxial, very nearly isotropic, N is 1.359(1) for λ = 589 nm, and birefringence is 0.0009. Electron microprobe analyses gave (wt%) Na = 23.4, Al = 13.9, F = 58.6, Li = 3.56 (calculated), with a total of 99.46. The empirical formula (based on 6 F atoms) is Na1.98Li1.00Al1.00F6. The crystal structure was not solved, presumably because of unit-cell scale twinning, but similarities to the perovskite-type structure exist. The mineral is named for William B. Simmons, Professor of Mineralogy and Petrology, University of New Orleans, New Orleans.

Bederite, a new pegmatite phosphate mineral from Nevados de Palermo, Argentina; description and crystal structure

Abstract Bederite, ideally ⃞Ca2Mn2+2Fe3+2Mn2+2(PO4)6(H2O)2, orthorhombic, a = 12.559(2), b = 12.834(1), c = 11.714(2) Å, V= 1887.8(4) Å3, Z = 4, space group Pcab, is a new mineral from the El Peñón pegmatite, Nevados de Palermo, Salta Province, República Argentina. The mineral occurs as rare ellipsoidal nodules (~5 cm in diameter) enclosed in potassium feldspar or quartz at the core-margin zone of a beryl-type rare-element pegmatite. Associated minerals are quartz, potassium feldspar, muscovite, beryl, columbite, possibly heterosite, and powdery coatings of Mn- and Fe-oxides; in the dumps of the pegmatite, there are numerous other phosphates including altered triphylite-lithiophyllite, arrojadite, eosphorite, laueite, brazilianite, and fairfieldite. Bederite is very dark brown to black with a dark olive-green streak and a vitreous luster. It is brittle with an irregular fracture and a good cleavage parallel to {100}, Mohs hardness is 5, and the observed and calculated densities are 3.48(1) and 3.50 g/cm3, respectively. In transmitted plane-polarized light, bederite is pleochroic X = Y = olive green, Z = brown with X = Y > Z and X = a, Y = c. Z = b. In cross-polarized light, it is biaxial negative with strong dispersion, v > r, 2V(obs) = 54° and 2V(calc) = 60°. Refractive indices are as follows: α = 1.729(3), β = 1.738(3), γ= 1.741(3). Chemical analysis by electron microprobe plus the Penfield method and thermogravimetry gave P2O5 41.76, Al2O3 0.82, Fe2O3 12.00, FeO 2.25, MnO 20.59, MgO 3.45, ZnO 0.40, CaO 10.91, SrO 0.43, Na2O 0.63, H2O 3.52, sum 96.76 wt% where the Fe2O3 and FeO contents were derived from the refined crystal structure. The five strongest lines in the X-ray powder diffraction pattern are as follows: d Å), I, (h k l): 2.768,100, (4 0 2); 2.927, 78, (0 0 4); 3.006, 67, (1 4 1); 2.814, 35, (0 4 2); 2.110, 33, (1 6 0). The crystal structure of bederite was refined to an R index of 2.8% based on 2530 observed (>5σF) reflections measured with MoΚα X-radiation. Bederite is isostructral with wicksite, grischunite, and an unnamed wicksite-like phase; it is related to wicksite by the substitutions Na⃞ + M2Fe3+ → NaNa + M2Mg, M1Mn2+ → M1Fe2+ and M3Mn2+ → M3Fe2+.

Granitic Pegmatites and their Minerals: a Tribute to Petr ČernÝ

Petr Cerný certainly needs no introduction to the students and professional scientists who study granitic pegmatites. Cernýs prolific career of research on these rocks has spanned over forty years, with important contributions in the areas of 1) the mineralogy, geochemistry and petrology of

GRANITIC PEGMATITES AND THEIR MINERALS: A SECOND TRIBUTE TO PETR CERNY

Petr Cerný began his scientific carreer in the mid-1950s. From the beginning, he focused on granitic pegmatites in his home country, Czechoslovakia. He was particularly interested in granitic pegmatites enclosed in compositionally contrasting rocks, like serpentinites, which are common components