Crystal Structure Refinement and Mössbauer Spectroscopy of an Ordered, Triclinic Clinochlore

Abstract

Abstract_---The crystal structure of a natural, ordered llb-4 triclinic clinochlore has been refined in space group C1 from 4282 unique X-ray intensity measurements of which 3833 are greater than 3 times the statistical counting error (3~). Unit cell parameters are a = 5.3262(6) ,~; b = 9.226(1) A; c = 14.334(3) ,~; c~ = 90.56(2)~ [~ = 97.47(2)~ and ~/ = 89.979(9) ~ which represents the greatest deviation from monoclinic symmetry yet recorded for a triclinic chlorite. The final weighted R is 0.059 for reflections with I > 3(r and 0.064 for all reflections. The chemical formula is (Mgo966Fe0034)Ml(Mg0%2Fe0038)M22(Si296Alt04)Ot0 (OH)2(Mg0996Fe0.004)M32(A10841Fen~0102Cr00o4Ti00o4)M4(OH)6, which is consistent with electron microprobe (EMP), wet chemical analyses, Mrssbauer spectroscopy and X-ray structure refinement. The high degree of ordering of the divalent versus trivalent octahedral cations in the interlayer is noteworthy, with Fetn and A1 in M4 and virtually no Fe in M3. In the 2:1 layer, M1 and M2 each contain similar amounts of Fe. The 2 tetrahedral sites have nearly identical mean oxygen distances and volumes, and thus show no evidence of long-range cation ordering.

Quantitative texture analysis of glaucophanite deformed under eclogite facies conditions (Sesia-Lanzo Zone, Western Alps): comparison between X-ray and neutron diffraction analysis

“…A representative sample of M26 glaucophanite (1 cm 3) was powdered with an agate mortar and a powder diffraction pattern was collected using Cu Kc~ radiation, from 5 ° to 70 °, in steps of 0.01 ° and 3 seconds per step. A quantitative phase analysis using the Rietveld method was performed using a richterite (Hawtorn et al 1997) and a clinochlore (Smyth et al 1981) as starting structural models. The atomic fractions were deduced from chemical analysis.…”

High-pressure behavior of clinochlore

“…Interlayer bonding involves only hydrogen bonds; onethird of the atoms in chlorite are involved in hydrogen bonding, and it is this major structural feature that is the subject of the present study. The long-and short-range cation-ordering behavior of chlorites is now well understood from recent X-ray and spectroscopic studies (e.g., Rule and Bailey 1987;Welch et al 1995;Smyth et al 1997), which have shown that [6] Al is at M4 and [6] Mg is ordered at M3 (M3:M4 = 2:1). In end-member clinochlore, M1 and M2 sites of the 2:1 layer are filled by [6] Mg. A 29 Si MAS NMR study of synthetic monoclinic (C2/m) end-member clinochlore by Welch et al (1995) showed that there is a high degree of Al-Si short-range order within the tetrahedral sheet, as is also found in phlogopite and related micas (Herrero et al 1987;Circone et al 1991).…”

Luminescence and other spectroscopic properties of purple and green Cr-clinochlore

“…in both studied crystals occurs in tetrahedral sites, but in purple chlorite it is also present in the octahedral sheet. Doublet D4 has IS and QS parameters that are characteristic of the M4 site (Smyth et al 1997 ) a lower quadropole splitting means more distortion around a site, the measured doublets D1 were ascribed as originating from the M1 site. However, the majority of ferrous ions occupy M2 sites in purple and green Cr-clinochlore.…”

“…ions are present in M1 and M2 crystal sites, and Fe 3? ions are present in M3 and M4 crystal sites, as well as in a tetrahedral coordination (Goodman and Bain 1979, Blaauw et al 1980, Dyar 1987, deGrave et al 1987, Gregori and Mercader 1994, Smyth et al 1997. The influence of Al 3?…”