Distribution of chromium among the octahedral sites in chromian epidote from Iratsu, central Shikoku, Japan

Abstract

The crystal structures of chromian epidotes from the Sambagawa metamorphic rocks, central Shikoku, Japan, were refined using single crystal data to investigate the distribution of chromium among the three independent octahedral sites. On the basis of the electron probe microanalysis of the chromian epidotes on the polished surface of thin sections, three chromian epidote grains in a thin section were selected for X ray intensity data collection, and their Cr 2 O 3 values were as much as 1.79, 4.99, and 4.74 wt%, respectively, which led to values of 0.113, 0.315, and 0.299 Cr apfu, respectively. Nevertheless, their Fe 2 O 3 contents were nearly constant (8.1 8.7 wt%, 0.49 0.52 Fe apfu) and their SrO contents were not significant (1.8 3.2 wt%, 0.08 0.15 Sr apfu). The selected chromian epidote grains were picked out using a binocular optical microscope from the thin sections, and were labeled as CrEp1, CrEp2, and CrEp3, respectively. The oxidation state of chromium was found to be trivalent by the optical spectrum. were refined using 1145, 1853 and 1438 unique reflections, respectively, and the calculations converged at R factors of 5.8%, 6.6%, and 3.2%, respectively. The resulting structural formula were: [Ca] A1 [Ca 0.86 (Dollase, 1969(Dollase, , 1971Kvick et al., 1988;Ferraris et al., 1989;Bonazzi et al., 1990Bonazzi et al., , 1992 Menchetti, 1994, 1995;Langer et al., 2002;Nagashima and Akasaka, 2004; and others) and a variety of spectroscopic methods (Burns and Strens, 1967;Dollase, 1973;Paesano et al., 1983;Fehr and Heuss Aßbichler, 1997;Langer et al., 2002; and others). On the other hand, the distribution of Cr 3+ has been studied by means of optical spectroscopy (Burns and Strens, 1967), but the structural refinement of chromian epidote has not been carried out so far, because the occurrence of chromian epidote is not common.Recently, Nagashima et al. (2006) reported on the occurrence of chromian epidote containing up to 6 wt% Cr 2 O 3 in pebbles of omphacite rock from the Sambagawa metamorphic rocks in central Shikoku, Japan. In the present study, we have investigated the structure of chromian epidote obtained using single crystal X ray diffraction coupled with 57 Fe Mössbauer spectroscopy and optical spectroscopy. EXPERIMENTAL METHODS SamplesThe chromian epidote studied occurs in omphacite rock pebbles obtained from the Sambagawa metamorphic rocks in central Shikoku, Japan. Nagashima et al. (2006) distinguished three types of chromian epidote: Ca epidote with < 3 wt% SrO, Sr rich epidote with 3 11 wt% SrO, and the rare earth elements (REE) rich epidote with 2 7 wt% (Ce 2 O 3 + La 2 O 3 + Nd 2 O 3 + Pr 2 O 3 + Sm 2 O 3 + Gd 2 O 3 ), and described the cores of the Sr rich epidote being overgrown with Ca epidote, whereas the Ca epidotes were rimmed by and/or intergrown with REE rich epidotes. The Cr 2 O 3 content in the epidote is variable and attains up to 6 wt% around the chromite grains. The Cr distribution is not related to any zonal structure. The Fe 3+ content varies within a narrow range: 5...

Vanadian and chromian garnet- and epidote-supergroup minerals in metamorphosed Paleozoic black shales from Čierna Lehota, Strážovské vrchy Mountains, Slovakia: crystal chemistry and evolution

“…Vanadium usually enters the structure of epidote-supergroup minerals as V 3+ because only an extremely oxidizing environment allows V 5+ stabilization in epidote-supergroup minerals (Gieré and Sorensen, 2004). The behaviour of Cr in the epidote-supergroup minerals structure is unclear due to the possible Al–Cr disorder between the M 1 and M 3 sites (Armbruster et al , 2006, Nagashima et al , 2007). It is even more enigmatic in the light of previous research on Cr-rich clinozoisite from Finland and Japan with presumed exsolution of a V- and Cr-bearing phase from a clinozoisite structure (Nagashima et al , 2011).…”

“…However, the abundance of octahedral cations other than Al, including Fe 2+ , Mn 2+ and V in some compositions of allanite-(La) from Čierna Lehota, suggests that V could be distributed in both M 1 and M 3 sites. This presumption is based on the geometry of the octahedral sites, where the M 3 site is larger and more distorted than both M 1 and M 2 sites (Dollase, 1968) and also on REE occupancy at the A 2 site, which requires divalent cations in the M 3 octahedra (Nagashima et al , 2007). Vanadium, which is smaller (0.64 Å) than Fe 2+ (0.78 Å) and Mn 2+ (0.83 Å), is the most probable substituent for Al 3+ (0.535 Å, radii from Shannon, 1976).…”

Crystal chemistry of Sr–rich piemontite from manganese ore deposit of the Tone mine, Nishisonogi Peninsula, Nagasaki, southwest Japan

“…and T is tetrahedral sites (Armbruster et al 2002;Mills et al, 2009). Epidote-group minerals of epidote-supergroup, such as clinozoisite, epidote and piemontite, are important Sr containers in metamorphic rocks (e.g., Grapes and Watanabe, 1984;Mottana, 1986;Reinecke, 1986;Nagasaki and Enami, 1998;Enami, 1999;Miyajima et al, 2003;Nagashima et al, 2006), metamorphosed manganese ore deposits (e.g., Kato and Matsubara, 1986;Bonazzi et al, 1990;Perseil, 1990;Minakawa, 1992;Nagashima et al, 2007;Minakawa et al, 2008; A d v a n c e P u b l i c a t i o n A r t i c l e al., 2010), and metamorphosed manganiferous iron ore deposits (Akasaka et al, 1988;Togari et al, 1988). Thus, the occurrence and crystal chemical properties of Sr-bearing epidote-group minerals, such as solubility of Sr and its effect on the Mn and Fe contents and crystal structure, have been interested in.…”

Estudo de propriedades de luminescência, de ressonância paramagnética eletrônica e de centros de cor da pumpelita e de sua correlação com defeitos pontuais.

“…A figura 5.25 mostra que somente acima de 800 °C a cor verde é também absorvida, deixando mais avermelhado o mineral. A partir da suposição que o Cr 3+ ocupa um ambiente octaédrico e, usando o diagrama de níveis de energia devido a esta coordenação octaédrica, foi possível identificar as bandas em 340, 460, 470, 660 e 680 nm devido a Cr 3+ e estes resultados concordam com o trabalho realizado porNagashima (2007). Na figura 5.28, no espectro obtido subtraindo dos espectros com irradiação do espectro da amostra natural foi possível identificar as bandas em 455 nm, 550 nm, 733 nm e 833 nm devido a Mn 3+ é o que confirma o spin observado porBurns (1967).Por outro lado, na figura 5.26, os espectros de AO de amostras recozidas em 600 °C, 700 °C e 900 °C mostram num resultado muito interessante, a banda em torno de 1000 nm, devida a, Fe 2+ , muda pouco com o tratamento térmico até 800 °C (não mostrado aqui o tratamento térmico em 800 °C), mas, entre 800 °C e 900 °C a banda decresce.…”