Chemical and structural study of 1M- and 2M 1-phlogopites coexisting in the same Kasenyi kamafugitic rock (SW Uganda)

3T-phlogopite from Kasenyi kamafugite (SW Uganda): EPMA, XPS, FTIR, and SCXRD study

“…It is noteworthy that IV Fe 3+ , estimated by the formula <T-O> (Å) = 1.607 + 4.201⋅10 −2 Al + 7.68⋅10 −2 Fe (with Al and Fe in atoms per formula unit) given in Brigatti and Guggenheim (2002), provides 0.08 apfu, which compares well with the value estimated from Mössbauer spectroscopy. The average octahedral distances (Table 4) in the 3T phlogopite are similar to those found for the 1M and 2M 1 polytypes from the same BU1 rock [<M1-O> = 2.084 and 2.083 Å, respectively, <M2-O> = 2.068 and 2.069 Å, respectively, see Lacalamita et al (2012)]. The final octahedral cation distribution was obtained by also considering that the so called "Shift M2 " parameter (see Table 6) is known to be proportional to the oxy component (Lacalamita et al 2011).…”

“…Specifically, the infrared absorption in the OH --stretching region (∼3740-3600 cm -1 ) for the studied 3T-BU1 sample is shown in comparison to those of the rock coexisting 1M-and 2M 1 -polytypes, which were very recently investigated (see Lacalamita et al 2012). The IR absorption signals are very similar, as expected for chemically homogeneous 1M-, 2M 1 -, and 3T-BU1 micas.…”

“…In addition, the near surface Fe 3+ /Fe 2+ ratio, as determined by Lacalamita et al (2012); t tet : tetrahedral sheet thickness calculated from z coordinates of basal and apical O atoms; TQE: tetrahedral quadratic elongation (Robinson et al 1971); TAV: tetrahedral angle variance (Robinson et al 1971); τ: tetrahedral flattening angle; α: tetrahedral rotation angle (Hazen and Burnham 1973); ∆z: departure from complanarity of the basal O atoms (Güven 1971); D.M. : dimensional misfit between tetrahedral and octahedral sheets (Toraya 1981); Ψ: octahedral flattening angles (Donnay et al 1964a(Donnay et al , 1964b; BLD: bond-length distortions (Renner and Lehmann 1986); ELD: edge-length distortion (Renner and Lehmann 1986); OQE: octahedral quadratic elongation (Robinson et al 1971); OAV: octahedral angle variance (Robinson et al 1971); e u , e s : mean lengths of unshared and shared edges, respectively (Toraya 1981); Shift M : off-center shift of the M cation calculated using the IVTON software (Balić-Žunić and Vickovic 1996); t oct : octahedral sheet thickness (Toraya 1981); t int calculated from the z coordinates of basal O atoms; Δ K-O = <K-O> outer -<K-O> inner ; t K-O4 : projection of K-O4 distance along c*.…”

“…In the present work, a 3T phlogopite from Kasenyi kamafugitic rock, SW Uganda, has been investigated by integrating single-crystal X-ray diffraction results (SCXRD) and electron probe microanalyses (EPMA) with micro-Fourier transform infrared (micro-FTIR) and X-ray photoelectron spectroscopy (XPS) inferences, to: (1) study in depth its crystal-chemical peculiarities and (2) shed light on similarities and/or differences between the 3T-and the 1M-and 2M 1 -micas from the same rock, which were recently reported in the literature (see Lacalamita et al 2012).…”

2M1-phlogopite from Black Hills (South Australia): The first case of configurational polytype in micas

“…In a very recent paper on the coexistence of 1M-and 2M 1 -polytypes, it was hypothesized that the probability of occurrence of 2M 1 polytype depends on the degree of cation ordering at octahedral sites but may be also related to the extent of oxy-type substitutions that destabilize the octahedral sheet . All the 2M 1 -phlogopites reported to date in literature are completely (Ohta et al 1982;Laurora et al 2007; BU1_8, BU1_14, and BU1_16 crystals from Lacalamita et al 2012) or partially (Takeda and Ross 1975;Bigi and Brigatti 1994;Brigatti et al 2008; BU1_15 and BU1_17 crystals from Lacalamita et al 2012) disordered at octahedral sites. Also the 1M-BHG polytype coexisting with the study sample show disordered cation distribution at octahedral sites, apart from the partial ordering (Ti at M2 site) directly caused by the Ti-oxy substitution.…”