Controversial X-ray Diffraction, Mössbauer, FTIR and Raman Results on Magnesiocarpholite

Yves Fuchs (fuchs@lmcp.jussieu.fr)¹, Marcello Mellini (mellini@unisi.it)² & Isabella Memmi (memmi@dst.unisi.it)²

¹ case 115, LPMC, Université Pierre et Marie Curie, 4 Place Jussieu, 75252 PARIS cedex05, FRANCE

² Dipartimento di Scienze della Terra, Via Laterina 8, 53100 Siena, Italy

Space group of carpholite, Ccca, has been matter of debate (Ferraris et al, 1992). We reinvestigated the problem, characterizing magnesiocarpholite from two occurrences (ML, Monti Leoni, and MA, Monte Argentario, Tuscany, Italy) by diffraction and spectroscopy techniques. The crystal structure analyses, led to R = 0.02, do not give indication of lower symmetry. Both the refined models produce reliable data fitting what previously reported; in particular, the small residual peaks occurring in the final difference Fourier syntheses match the so-called K-site of nonstoichiometric carpholites (Ghose et al., 1989). ML and MA carpholites have similar compositions, with MA iron-richer (X_Mg=0.64) than ML (X_Mg=0.66); they also have comparable K (0.002-0.004 a.p.f.u.) and F (0.60-1 wt%) contents. Mössbauer spectra indicate: a) small Fe³⁺ content in both samples (8.7% in MA and 3.8% in ML); b) no Fe³⁺ in tetrahedral site; c) only one Fe octahedral site; d) equivalent quadrupole splitting parameters in ML and MA, possibly indicative for similar Fe environments. Controversial results derive from FTIR and Raman spectroscopy. The spectra of the two carpholite samples are very different in the OH region. MA shows five (3570, 3580-3590, 3620-3630, 3650 and 3690-3700 cm^-1), ML only three (3570, 3580-3590, and 3620-3630 cm^-1) bands plus a very weak FTIR band at 3325 cm^-1. These bands were attributed to OH groups facing local octahedral arrangements with different occupancies. FTIR shows further differences: in MA, broadening of the 1084 cm^-1 band, indicative for variable Si-O distances; larger and stronger 995-1005 cm^-1 absorption. OH bending vibrations (610 and 635 cm^-1) occur in both samples but MA reveals also a strong absorbance band at 538 cm^-1 (Al-O(H) bending) absent in ML. At difference from X-rays, spectroscopy suggests higher symmetry in ML magnesiocarpholite. The conflicting results may be rationalized in terms of short- and long-range ordering.

Ferraris G, Ivaldi G & Goffè B, N. Jb. Miner. Mh, 8, 337-347, (1992).

Ghose S, Sen Gupta PK, Boggs RC & Schlemper EO, American Mineralogst, 74, 1084-1090, (1989).

Hydrothermal Synthesis of Axinite and Equilibrium Conditions with Ca-Tourmaline (uvite)

Yves Fuchs (fuchs@lmcp.jussieu.fr) & Tolga Oyman

Lab. Min. Cristall, case 115,UPMC, 4 pl. Jussieu, 75252 Paris cedex05, France

Axinite Ca₂(Mn,Fe)Al₂BSi₄O₁₅(OH). is commonly found as geode mineral in different environments like metabasalts (Bergen area, Norway) or granite (Priatu, Sardinia, Italy) but skarns and associated ore deposits represent the most important type of occurrences ( Siberia, Yukon, Montana, Turkey etc). Hydrothermal synthesis of axinite and Sn-axinite was performed by I. Ya. Nekrasov and G.A. Kashirtseva (1975) at T from 300 to 500°C and P_H20 = 30 to 100 MPa using oxides and boric acid. In order to study the equilibrium conditions of axinite, uvite (Ca-tourmaline) and other skarn minerals hydrothermal synthesis was performed in gold sealed capsules at T from 400° to 600°C, pressures from 40 to 70 Mpa using epidote+boric acid, epidote+boric acid+albite or anorthite or plagioclase mixtures of various composition. Duration of the experiments was 270 days at 400°C, 70 days at 500° and 30 days at 600°C. Products of synthesis were characterised by XRD. Chemical analyses of the synthetic minerals were performed using MET. Experiments with only epidote or epidote+anorthite as primary minerals produced axinite at 400 and 500°C. The formation of Ca-tourmaline was related to Na/Ca ratio in the primary mixture. At 400°C a mixture of epidote+brucite+albite+boric acid produced coexistent axinite+uvite+plagioclase. On the basis of the first results it seems that distribution of axinite and uvite in contact metamorphism is related to T and to the Na/Ca ratio and that axinite is the epidote-equivalent in metamorphic domain, when B amount is sufficient.

Nekrasov I. Ya. & Kashirtseva G. A., Dok. Akadem. Nauk SSSR, 222(2), 440-443, (1975).

Spectroscopic Investigations of Serpentine Minerals

Yves Fuchs (fuchs@lmcp.jussieu.fr)¹, Celine Lemaire¹, Marcello Mellini (mellini@unisi.it)² & Cecilia Viti²

¹ case 115, LPMC, Université Pierre et Marie Curie, 4 Place Jussieu, 75252 PARIS cedex05, France

² Istituto Scienze della Terra, Via Laterina, Siena, Italy

Pure, monomineralic vein samples from Elba island, Italy (lizardite-1T MFN3, antigorite 7 and 18, with superperiodicities of 38 and 49 Å, respectively) have been investigated by spectroscopic methods. Mössbauer data for lizardite-1T MFN3 indicate the occurrence of octahedral ferrous iron, octahedral ferric iron and tetrahedral ferric iron (59.9, 31.3 and 8.8% of total iron, respectively); only one octahedral site is present in lizardite, in agreement with previous X-ray structure refinement. In antigorite, iron mostly occur as Fe²⁺(88.6% vs 11.4% Fe³⁺ in sample 7 and 83.2% Fe²⁺ vs 16.8% Fe³⁺ in sample 18): both are present in the octahedral site. No evidence for the occurrence of Fe³⁺ in tetrahedral site has been detected. Mössbauer data indicate different conditions for the lizardite and the antigorite. This reflects a general crystal-chemical behaviour: lizardite is always characterized by a greater amount of trivalent cations with respect to antigorite and chrysotile. Most significant infrared results arise from the comparison between lizardite and antigorite spectra in the stretching and bending vibrations of the tetrahedral sheet. The peak at 1086 cm^-1 in lizardite and those at 1078 and 1082 cm^-1 in antigorite (7 and 18, respectively) are attributed to the apical Si-O bond: in this region, IR spectra suggest a similar structural arrangement and similar apical bond distances. Peaks in the 990 - 950 cm^-1 are attributed to the stretching vibrations of the basal Si-O bonds. Lizardite is characterized by a sharp peak at 950 cm^-1, with a shoulder at 992 cm^-1, whereas antigorite shows a sharp peak at ~ 985 cm^-1 (986 and 983 cm^-1, in samples 7 and 18, respectively, with shoulders at 967 cm^-1): the lower wavenumber in lizardite would indicate lower energy Si-O bridging bonds with respect to antigorite. The 565 band (attributed to Si-O bending) is evident in the antigorite spectra, whereas it is absent in lizardite. This different behaviour is explained as due to the different symmetry: bending of Si-O is possible only in monoclinic antigorite, but not permitted in trigonal lizardite, since this bending would destroy the local three-fold symmetry.