Abstract In the present work, crystal chemical variations between 1M and 2M1 phlogopites coexisting in the same rock sample from kamafugite of Kasenyi (southwest Uganda, west branch of the East African Rift) were explored by electron probe microanalyses, single crystal X-ray diffraction and Mo¨ ssbauer spectroscopy. Chemical analyses revealed close similarity both within and between the two polytypic arrangements as well as high TiO2 (*4.9 wt%) and Al2O3 (*12.9 wt%), and low Cr2O3 (*0.8 wt%), F (*0.3 wt%) and BaO (*0.2 wt%) con- tents. Room temperature 57Fe Mo¨ ssbauer investigation proved that the studied mica is a tetraferriphlogopite with: IVFe3? = 19(1) %, VIFe2? = 58(1) %, VIFe3? = 23(1) %. Single crystal refinement showed that both polytypes have narrow range of variation in terms of some relevant unit cell parameters and similar values in terms of mean bond lengths, mean atomic numbers and distortion parameters. Similar substitutions were active in the structure of the 1M and 2M1 studied phlogopites. However, in 2M1 poly- types the oxy-type substitutions were found to occur to a greater extent. Comparison of unit layer of 1M mica (in the 2M1 setting) with that of the 2M1 ones showed that the 2M1 polytypes are affected to different extent by relative shifts of the upper and lower triads of octahedral oxygens along the ±b directions. This effect did not cause any symmetry lowering in the T-O-T layer of the studied samples.
CHEMICAL AND STRUCTURAL STUDY OF 1M- AND 2M1-PHLOGOPITES COEXISTING IN THE SAME KASENYI KAMAFUGITIC ROCK (SW UGANDA)
LACALAMITA M;MESTO, ERNESTO;SCHINGARO, Emanuela
2012-01-01
Abstract
Abstract In the present work, crystal chemical variations between 1M and 2M1 phlogopites coexisting in the same rock sample from kamafugite of Kasenyi (southwest Uganda, west branch of the East African Rift) were explored by electron probe microanalyses, single crystal X-ray diffraction and Mo¨ ssbauer spectroscopy. Chemical analyses revealed close similarity both within and between the two polytypic arrangements as well as high TiO2 (*4.9 wt%) and Al2O3 (*12.9 wt%), and low Cr2O3 (*0.8 wt%), F (*0.3 wt%) and BaO (*0.2 wt%) con- tents. Room temperature 57Fe Mo¨ ssbauer investigation proved that the studied mica is a tetraferriphlogopite with: IVFe3? = 19(1) %, VIFe2? = 58(1) %, VIFe3? = 23(1) %. Single crystal refinement showed that both polytypes have narrow range of variation in terms of some relevant unit cell parameters and similar values in terms of mean bond lengths, mean atomic numbers and distortion parameters. Similar substitutions were active in the structure of the 1M and 2M1 studied phlogopites. However, in 2M1 poly- types the oxy-type substitutions were found to occur to a greater extent. Comparison of unit layer of 1M mica (in the 2M1 setting) with that of the 2M1 ones showed that the 2M1 polytypes are affected to different extent by relative shifts of the upper and lower triads of octahedral oxygens along the ±b directions. This effect did not cause any symmetry lowering in the T-O-T layer of the studied samples.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
