2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 196-6
Presentation Time: 9:40 AM


CEMPÍREK, Jan1, GREW, Edward S.2, KAMPF, Anthony R.3, MA, Chi4, NOVÁK, Milan1, GADAS, Petr1, ŠKODA, Radek1, VAŠINOVÁ-GALIOVÁ, Michaela5, PEZZOTTA, Federico6 and GROAT, Lee A.7, (1)Department of Geological Sciences, Masaryk University, Brno, 611 37, Czech Republic, (2)School of Earth and Climate Sciences, University of Maine, 5790 Bryand Global Research Center, Orono, ME 04469, (3)Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, (4)Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, (5)Department of Chemistry, Masaryk University, Brno, 611 37, Czech Republic, (6)Mineralogy Department, Museo di Storia Naturale, Corso Venezia 55, Milan, I-20121, Italy, (7)Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, BC V6T 1Z4, Canada, esgrew@maine.edu

The system B2O3-Al2O3-SiO2 (BAS) includes two ternary phases occurring naturally, boromullite, Al9BSi2O19, and boralsilite, Al16B6Si2O37, as well as synthetic compounds structurally related to mullite such as Al16B4Si4O38. A potentially new anhydrous ternary BAS mineral structurally akin to boralsilite, but distinct in composition, occurs with albite and K-feldspar as a breakdown product of spodumene in the elbaite-subtype Manjaka granitic pegmatite, Sahatany Valley, Madagascar, where it forms rare intergrowths of subparallel prisms up to 100 μm long. Optically, the mineral is biaxial (–), α = 1.607(1), β = 1.634(1), γ = 1.637(1) (white light), 2Vx(calc.) = 36.4°, X ≈ c; Y ≈ a; Z = b. An averaged analysis by EMP and LA-ICP-MS (Li, Be) gives (wt%) SiO2 20.24, B2O3 11.73, Al2O3 64.77, BeO 1.03, MnO 0.01, FeO 0.13, Li2O 1.40, Sum 99.29. Raman spectroscopy in the 3000–4000 cm−1 region rules out the presence of significant OH or H2O. The mineral is monoclinic, space group I2/m, a = 10.3832(12), b = 5.6682(7), c = 10.8228(12) Å, β = 90.106(11)°; V = 636.97(13) Å3, Z = 1. In the structure, R1 = 0.0416 for 550 Fo > 4σ(Fo), AlO6 octahedral chains //[010] are cross-linked by Si2O7 disilicate groups, BO3 triangles, and chains of AlO4 and two AlO5 polyhedra. The last, Al4 and Al5, cannot be occupied simultaneously; the refinement gives 54% and 20% occupancy, respectively. Bond valence sums for Al4 suggest Li is likely to be sited here, whereas Be could be at Al5. One of the 9 O sites is only 20% occupied; this O9 site completes the coordination of Al5 and also represents the 4th corner of a partially occupied BO4 tetrahedron, in which case the B site is shifted out of the plane of the BO3 triangle. If all sites were filled (Al4 and Al5 to 50%), the formula becomes Al16B4Si4O38, close to Li1.08Be0.47Fe0.02Al14.65B3.89Si3.88O36.62 calculated from the analyses assuming cations sum to 24. The compatibility index based on the Gladstone – Dale relationship is 0.001 (“superior”). Boralsilite and the new mineral are inferred to have grown in disequilibrium conditions resulting from a combined pressure+chemical quench. Since Li contents of the new mineral average 7 times that of boralsilite, Li released by spodumene breakdown could have created a suitable environment for a second boralsilite-like mineral to crystallize.