MnAl₂Si₂O₈ Solid Solution in Synthetic (K,Ba)-Feldspars

Tomoaki Matsui (matsui@edu.kagoshima-u.ac.jp)¹, Mitsuyoshi Kimata (kimata@arsia.geo.tsukuba.ac.jp)², Shizuo Saito (ssaito@ims.tsukuba.ac.jp)³ & Susumu Shimoda²

¹ Faculty of Education, Kagoshima University, Kagoshima, 890-0065, Japan

² Institute of Geoscience, University of Tsukuba, Ibaraki, 305-8571, Japan

³ Institute of Material Science, University of Tsukuba, Ibaraki, 305-8573, Japan

Mn-bearing (K,Ba)-feldspars along the KAlSi₃O₈-BaMnSi₃O₈ series were synthesized by heating chemical reagents, Al(OH)₃, K₂CO₃, MnCO₃ and SiO₂-gel under an N₂ atmospheric condition, and characterized by X-ray powder diffraction and chemical analyses. The same synthesis method as carried out in the synthesis of Mn-bearing anorthite (Matsui & Kimata, 1997) was applied to the series of title samples. The maximum limit of partial solubility of Mn²⁺ was about 1.45 wt% of MnO, which was much higher than the most Mn-rich one (0.34 wt% MnO) reported for natural hyalophane, (K,Ba)-feldspar (Frondel et al., 1966). No feldspar phase was confirmed for the samples from the syntheses along the KAlSi₃O₈-MnAl₂Si₂O₈ series. The crystal structures of four synthetic feldspars with chemical compositions of (K_0.656Ba_0.261Mn_0.024Al_0.004[ ]_0.055)(Al_1.242Si_2.762)O₈ ([ ] stands for a defect at M site), (K_0.370Ba_0.564Mn_0.012Al_0.024[ ]_0.030) (Al_1.618Si_2.406)O₈, (K_0.305Ba_0.619Mn_0.021Al_0.017[ ]_0.038)(Al_1.653Si_2.364)O₈ and (K_0.169Ba_0.753Mn_0.016Al_0.035[ ] _0.027) (Al_1.847Si_2.188)O₈, which are monoclinic, C2/m, Z = 4, have been refined to an R index of 4.6%, 5.7%, 3.3% and 4.1%, respectively. Their refinements confirmed that the Mn²⁺ occupy the M site in feldspar structures, nevertheless the EPMA data shows that Mn²⁺ can also occupy the T site of the feldspar. Moreover, it seems to that solid solution of the [ ]Si₄O₈ (excess silica component) and Al(Al₃Si)O₈ end-members contributes to the entrance of Mn²⁺ into the (K,Ba)-feldspar. It was clarified that the monoclinic (K,Ba)-feldspars were allowed to solve less amount of Mn²⁺ than the triclinic anorthite or albite structure. These crystal-chemical features in the synthetic Mn-bearing feldspars can provide a comprehensive explanation for the occurrence of Mn-rich feldspars in nature.

Matsui T & Kimata M, Eur. J. Min, 9, 333-344, (1997).

Frondel C, Ito J & Hendricks JG, Am. Min, 51, 1388-1393, (1966).

Formation of Potassium-Rich Clinopyroxenes and Carbonate-Silicate Melting Relations in the K₂(Ca, Mg)(CO₃)₂ - Clinopyroxene - Garnet System at 3.8 - 7.0 GPa: Modelling the Genesis of Diamond-Bearing Rocks of Kazakhstan Metamorphic Complex

Yuriy A. Matveev (yukob@gol.ru)¹, Ludmila T. Chudinovskikh (chud@iem.ac.ru)² & Yuriy A. Litvin (litvin@iem.ac.ru)²

¹ Moscow, 113461, Perekopskaya street, 27/2 - flat 35, Russia

² Institute of experimental Mineralogy, Russian Ac.Sci., Chernogolovka, Moscow Region, 142432, Russia, Russia

Experimental studies of melting relations of the carbonate - silicate system K₂(Ca,Mg)(CO₃)₂ - CaMgSi₂O₆ - (Mg,Ca)₃Al₂Si₃O₁₂ were carried out at 3.8 - 7.0 GPa to explore the version that diamonds of Kokchetav metamorphic complex (Kazakhstan) have magmatic origin. The version is based on the findings of diamond - bearing carbonate - silicate rocks composed of dolomite, potassium - rich clinopyroxene and pyrope - grossular garnet (Perchuk et al, 1995). Additional arguments deduced from high-pressure synthesis of potassium - rich clinopyroxenes (Harlow, 1997), discovery of potassium -rich fluid - carbonatitic inclusions in Kokchetav diamonds (De Corte et al, 1999), and experimental crystallization of diamonds in multicomponent carbonatitic melts of natural compositions (Litvin and Zharikov, 1999). It was found that clinopyroxenes with ultra-high potassium contents (up to 5,7 wt.% K₂O) formed at 7.0 GPa and 1500 - 1600 C in the course of carbonate - silicate interactions in the experimental system. Also interaction between ultra-K - carbonate and silicate melts at mantle conditions was investigated. The studies of melting relations of the K₂(Ca, Mg)(CO₃)₂ - clinopyroxene - garnet system showed that stable assembly of potassium - rich clinopyroxene, pyrope - grossular garnet, Ca-Mg-K - carbonate and coesite was formed in the near - solidus conditions. Taking into account that diamonds crystallize in K-Ca-Mg - carbonate melts oversaturated with carbon, it can be seen that the natural diamond - bearing carbonate - silicate assembly is reproducible in high-pressure experiment. This provides direct grounding in the magmatic version of diamond genesis in Kokchetav - type deposits, the complete geological history of which was influenced by scale processes of the mantle dynamics.

Perchuk LL, Yapaskurt VO, Okay A, Petrology, 3, 267-309, (1995).

Harlow GE, American Mineralogist, 82, 259-269, (1997).

De Corte K, Cartigny P, Shatsky VS, Geochimica Cosmochimica Acta, 62, 3765-3773, (1998).

Litvin YuA, Zharikov VA, Doklady Earth Sciences, 367a, 801-805, (1999).

Mechanism of Electrical Conductivity in Lower Mantle Perovskite

Catherine McCammon (catherine.mccammon @uni-bayreuth.de) & Yousheng Xu

Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany

Laboratory measurements of electrical conductivity of minerals relevant to the Earth's mantle combined with geophysical measurements of mantle conductivity can provide information not otherwise available on mantle properties, such as defect chemistry, oxygen fugacity and the presence of hydrogen. In the past, technical difficulties associated with measuring electrical conductivity at high pressures and temperatures have been largely responsible for conflicting results reported by different laboratories, but recent breakthroughs have produced datasets that largely agree with one another.

Previous in situ electrical conductivity measurements of silicate perovskite suggest that activation energies are larger at high temperature (0.6-0.9 eV) compared to low temperature (0.2-0.5 eV). There are important implications for the electrical conductivity of the lower mantle, since laboratory measurements must be extrapolated to higher temperatures. In order to explore this question further, we determined the temperature dependence of electrical conductivity in silicate perovskite at high pressure using complex impedance spectroscopy, and used Mössbauer spectroscopy to determine the concentration of ferric iron corresponding to the conductivity measurements.

Our data show that there is a significant change in the slope of log conductivity versus inverse temperature, and confirm pervious results that show steeper slopes at high temperatures. Possible explanations for the increase in slope are (1) a change in the mechanism of electrical conductivity at high temperature; and (2) a change in the density of charge carriers as a function of temperature. Using the Mössbauer data, we can estimate the number of charge carriers, and hence determine whether a change in the mechanism of electrical conductivity with temperature in silicate perovskite is consistent with the data.