Stability and Crystal Structure of a High Pressure Mixed-Chain Biopyribole

Jürgen Konzett (juergen.konzett@uibk.ac.at)¹, Hexiong Yang (yang@gl.ciw.edu)² & Stephanie L. Japel (sjapel@jhu.edu)³

¹ Institut für Mineralogie und Petrographie, Universität Innsbruck, Innrain 52, A-6020 Innsbruck, Austria

² Geophysical Laboratory, 5251 Broad Branch Road, NW, Washington, DC 20015, U.S.A.

³ Dept. of Earth and Planetary Sciences, Johns Hopkins University, Olin Hall, 3400 North Charles Street, Baltimore, MD 21218, U.S.A.

Phases that belong to the "biopyribole" polysomatic series may be regarded as having crystal structures composed of talc-like (M) and pyroxene-like (P) layer modules stacked along (010) (Thompson 1981). Biopyriboles that can be isolated as single crystals belong to two different stacking series: (1) M_(n-1)P with pyroxenes (P), amphiboles (MP) and wide-chain biopyriboles such as jimthompsonite (MMP), each containing single-, double-, or triple tetrahedral chains, and (2) M_nPM_(n-1)P with mixed-chain biopyriboles such as chesterite (MMPMP), containing double and triple tetrahedral chains. Naturally occurring wide- or mixed-chain biopyriboles have formed under low-P and low/medium-T conditions, mostly by (metasomatic) alteration of amphiboles and/or pyroxenes. Here we present data on the stability and crystallography of a high-P mixed-chain biopyribole (HMB) that was synthesized in peralkaline bulk compositions in the range 7-16 GPa and 1200-1400°C, representing the first known high-P/high-T mixed-chain biopyribole, capable of storing alkalis and water under upper mantle PT conditions. The HMB coexists with K-richterite, clinopyroxene and phase X (K_2-xMg₂Si₂O₇H_x) between 7 and 10 GPa, and with clinopyroxene and phase X between 13 and 16 GPa. At 5 GPa/1100°C, the HMB is unstable with respect to K-richterite, at 18 GPa/1300°C sodium garnet and a hydrous Na-Mg phase isostructural with aenigmatite appear as high-P breakdown products of the HMB. Structure analysis of a HMB single crystal synthesized at 10 GPa/1250°C yields the following unit-cell parameters: a = 9.8390(9) Å, b = 26.6471(6) Å, c = 5.2665(5) Å, ß = 106.25(1)°, V = 1325.6(4) ų with a composition K_1.10Na_2.32Ca_1.52Mg_5.85Al_1.23Si_12.04O₃₄(OH)₂. The HMB thus belongs to the M_nPM_(n-1)P series with n=1, containing single (pyroxene) and double (amphibole) tetrahedral chains with an MPP stacking sequence. TEM analysis reveals no stacking disorder at 10 GPa, and a structure refinement yields the following site occupancies: pyroxene slab: M1 Mg=0.46, Al=0.54; M2 Ca=0.25, Na=0.75; amphibole slab: M1 Mg=1.00; M2 Mg=0.97, Al=0.03; M3 Mg=1.00; M4 Mg=0.51, Na=0.44, K=0.05; A K=1.00; accordingly, the HMB formula can be written as 2[Ca_0.25Na_0.75)(Mg_0.46Al_0.54)Si₂O₆] x K_1.00(Ca_0.51Na_0.44K_0.05) (Mg_4.97Al_0.03)Si₈O₂₂(OH)₂.

Thompson JB, Rev. Mineral., Min. Soc. Am., 9A, 141-186, (1981).

Strain Dependent Non-Newtonian Rheology of Basaltic Melts

Anselm Koopmann, Ralf Buettner (buettner@ geologie.uni-wuerzburg.de) & Bernd Zimanowski

Physikalisch Vulkanologisches Labor, Institut für Geologie, Universität Würzburg, Pleicherwall 1, D-97070 Würzburg, Germany

The rheology of magmatic melt plays the major role during magma transport in conduits and also strongly influences the eruptive behavior. Whereas the non-newtonian characteristics of high silica melts (e.g. rhyolites and dacites) have already been experimentally determined, basaltic melts are mostly modelled to behave as a newtonian fluid. In this study we demonstrate non-newtonian behavior of basic and ultrabasic melt compositions even at very low viscosities.

At the Physikalisch Vulkanologisches Labor in Wuerzburg a high temperature rotational viscometer has been constructed, which allows high precision measurement of low viscosity silicate melts (remelted basic and ultrabasic rocks). This implies flow-field measurement over a wide range of strain rates, which is achieved by using large melt volumes, large spindle diameters, and an improved furnace geometry. All measurements were performed in the sub-liquidus regime in an argon atmosphere under atmospheric pressure.

We found that only for compositions with a phenocryst content less than 10 vol.% a newtonian description can be approximated. Irrespective of the viscosity all samples with a higher crystal content show a strongly strain rate dependent non-newtonian behavior. In this cases variations of the temperature dependent viscosity of more that one log magnitude were observed.

Synthesis and the Experimental Study of Sr-, Ba-, Rb- Containing Feldspar Solid Solutions

Andrey Kovalskii (kovalsky@iem.ac.ru)¹, Lubov Ogorodova (mineral@geol.msu.ru)², Anatoliy Chichagov (avchicha@issp.ac.ru)¹ & Alexey Kotelnikov (kotelnik@iem.ac.ru)¹

¹ Institute of Experimental Mineralogy, Chernogolovka, Moscow region, 142432, Russia

² Faculty of geology, Main Building, MSU, Vorobjovy Gory, Moscow, 119899, Russia

Three series of the (Na,Sr)-, (Sr,Ba)-, (K,Rb)- feldspar solid solutions were synthesized by the methods of dry sintering, hydrothermal recrystallization and cation-exchange with molten salts. The phase and chemical compositions of solid products were analysed by X-ray and microprobe methods, solutions and solid products were analysed by AAS - method. The unit cell parameters were refined for all synthesized series of solid solutions. Samples were recorded in the continuous scan mode of the full X-ray diffraction profile on a PC-HZG-4 automated diffractometer. Angular positions of reflections were estimated by the Spectr-8 program (A.V.Okhulkov, IEM RAS) with correction by the internal standard (spectrally pure Si, a=5.4305 Å). Unit cell parameters were refined by 30-60 reflections in the angular range of 7-39 degrees T by the LCC and PUDI programs (Burnham, 1991). The studies performed for (Na,Sr)- feldspars made it possible to refine substantially the point of the displacive transformation (at the 20°C) triclinic ­> monoclinic of Bambauer (Bambauer et al., 1984). This transformation takes place at the composition of solid solution (Na_0.72Sr_0.28)Al_1.28Si_2.72O₈ according to data of the present investigation. The unit cell parameters of the (K,Rb)- feldspar solid solutions were refined for members of both triclinic (Rb-microcline) and monoclinic (Rb-sanidine) series. The concentration correlations of the unit cell parameters for all synthesized series of feldspars were approximated by the polynomials. Thermochemical study of (Na,Sr)- and (Sr,Ba)- feldspars was performed by high temperature calorimetry of dissolvation at 973 K on a differential heat conducting Calvet microcalorimeter ("Setaram"). The calorimetric measurements gave enthalpies of dissolvation of solid solutions of (Na,Sr)- feldspars with X_Sr(Fsp)=0.0; 0.175; 0.51; 0.62 and 1.0. Calculated enthalpies of mixing (H_mix) allow to consider this system to be close to the ideal one with the negative deviation of H_mix from ideal. The results obtained on high-temperature dissolvation of solid solutions of (Sr,Ba)- feldspars with X_Ba(Fsp)=0.00; 0.19; 0.49; 0.52; 0.80 & 1.00 allow to consider the system to be ideal within measurement errors. It may be connected with the similar radii of the isomorphous ions (Sr,Ba) in the feldspars.

Burnham CW, Harvard University, Cambridge MA02138, 24, (1991).

Bambauer HU, Schops M, Pentinghaus H, Bull. Mineral, 107, 541-551, (1984).