Mt. Vulture trioctahedral micas-1M mainly consist of phlogopite-annite solid solutions with a little component of true micas (Schingaro et al., 2001 EJM, 13,1057). However, both Li-free, and Li- and F-rich compositions may coexist in the same volcano-stratigraphic level. In the present contribution two single crystals are considered, which are representative of both compositions. The crystals, labelled LC7-27o and LC7R1 (Rinaldi, 2004, Ph.D thesis) were studied by combining chemical determinations (EPMA, SIMS) and structural investigations (SCXRD). Electron Probe Micro-Analyses yielded XMg = 0.87, TiO2 = 1.07 wt% for sample LC7-27o, and XMg = 0.65, TiO2 = 5.39 wt% for sample LC7R1, while SIMS provided Li2O = 0.0046, F = 1.78, H2O =3.45 wt% for LC7-27o and Li2O = 0.140, F = 3.51, H2O = 2.17 wt% for LC7R1. Relevant structural data are the following: a = 5.346(1), b = 9.257(3), c = 10.312(3) Å, ? = 99.97°(2) for LC7-27o, and a = 5.310(1), b = 9.193(2), c = 10.096(3) Å, ? = 100.03°(3) for LC7R1. Anisotropic structure refinements in space group C2/m converged at R = 4.34, Rw = 4.39 and at R = 3.77, Rw = 4.09 for the Li-bearing and Li-free sample, respectively. When considering bond distances, both samples can be described as homo-octahedral whereas taking into account mean atomic numbers, LC7R1 is meso-octahedral. Significant differences occur between the two samples for some of the distorsion parameters commonly used for micas. In particular, LC7R1 displays higher values of BLDM(2), ShiftM(2) and lower values of ?K-O4 and tK-O4 than those of LC7-27o. These differences can be ascribed to the different extent of F- and Ti-substitutions affecting their structure. The final cation distribution was obtained by combining EPMA, SIMS and SCXRD data after taking into account the crystal chemical model illustrated by Scordari et al. (2004, 32nd IGC) and after careful analysis of the effect of several normalization schemes on mica formula recalculation. It turned out that in sample LC7-27o Ti is incorporated according to the Ti-vacancy mechanism, whereas more than one Ti-substitution occur in LC7R1.
An EMPA, SIMS and SCXRD study of phlogopites from Mt. Vulture (Potenza, Italy): considerations on cation partition
OTTOLINI L
2004
Abstract
Mt. Vulture trioctahedral micas-1M mainly consist of phlogopite-annite solid solutions with a little component of true micas (Schingaro et al., 2001 EJM, 13,1057). However, both Li-free, and Li- and F-rich compositions may coexist in the same volcano-stratigraphic level. In the present contribution two single crystals are considered, which are representative of both compositions. The crystals, labelled LC7-27o and LC7R1 (Rinaldi, 2004, Ph.D thesis) were studied by combining chemical determinations (EPMA, SIMS) and structural investigations (SCXRD). Electron Probe Micro-Analyses yielded XMg = 0.87, TiO2 = 1.07 wt% for sample LC7-27o, and XMg = 0.65, TiO2 = 5.39 wt% for sample LC7R1, while SIMS provided Li2O = 0.0046, F = 1.78, H2O =3.45 wt% for LC7-27o and Li2O = 0.140, F = 3.51, H2O = 2.17 wt% for LC7R1. Relevant structural data are the following: a = 5.346(1), b = 9.257(3), c = 10.312(3) Å, ? = 99.97°(2) for LC7-27o, and a = 5.310(1), b = 9.193(2), c = 10.096(3) Å, ? = 100.03°(3) for LC7R1. Anisotropic structure refinements in space group C2/m converged at R = 4.34, Rw = 4.39 and at R = 3.77, Rw = 4.09 for the Li-bearing and Li-free sample, respectively. When considering bond distances, both samples can be described as homo-octahedral whereas taking into account mean atomic numbers, LC7R1 is meso-octahedral. Significant differences occur between the two samples for some of the distorsion parameters commonly used for micas. In particular, LC7R1 displays higher values of BLDM(2), ShiftM(2) and lower values of ?K-O4 and tK-O4 than those of LC7-27o. These differences can be ascribed to the different extent of F- and Ti-substitutions affecting their structure. The final cation distribution was obtained by combining EPMA, SIMS and SCXRD data after taking into account the crystal chemical model illustrated by Scordari et al. (2004, 32nd IGC) and after careful analysis of the effect of several normalization schemes on mica formula recalculation. It turned out that in sample LC7-27o Ti is incorporated according to the Ti-vacancy mechanism, whereas more than one Ti-substitution occur in LC7R1.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
