Distribution of Fe among octahedral sites and its effect on the crystal structure of pumpellyite

Abstract

Two pumpellyites with the general formula W 8 X 4 Y 8 Z 12 O 56-n (OH) n were studied using 57 Fe Mo¨ssbauer spectroscopic and X-ray Rietveld methods to investigate the relationship between the crystal chemical behavior of iron and structural change. The samples are ferrian pumpellyite-(Al) collected from Mitsu and Kouragahana, Shimane Peninsula, Japan. Rietveld refinements gave Fe(X):Fe(Y) ratios (%) of 41.5(4):58.5(4) for the Mitsu pumpellyite and 46(1):54(1) for the Kouragahana pumpellyite, where Fe(X) and Fe(Y) represent Fe content at the X and Y sites, respectively. The Mo¨ssbauer spectra consisted of two Fe 2+ and two Fe 3+ doublets for the Mitsu pumpellyite, and one Fe 2+ and two Fe 3+ doublets for the Kouragahana pumpellyite. In terms of the area ratios of the Mo¨ssbauer doublets and the Fe(X):Fe(Y) ratios determined by the Rietveld refinements, Fe 2+ (X):Fe 3+ (X): Fe 3+ (Y) ratios are determined to be 22:14:64 for the Mitsu pumpellyite and 27:8:65 for the Kouragahana pumpellyite. By applying the Fe 2+ :Fe 3+ -ratio determined by the Mo¨ssbauer analysis and the site occupancies of Fe at the X and Y sites given by the Rietveld method together with chemical analysis, the resulting formula of the Mitsu and Kouragahana pumpellyites are established as Ca 8 (Fe 0.88 2+ Mg 0.68 Fe 0.77 3+ Al 1.66 ) R3.99 (Al 5.67 Fe 2.34 3+ ) R8.01 Si 12 O 42.41 (OH) 13.59 and Ca 8 (Mg 1.24 Fe 0.65 2+ Fe 0.46 3+ Al 1.66 ) R4.01 (Al 6.71 Fe 1.29 3+ ) R8.00 Si 12 O 42.14 (OH) 13.86, respectively. Mean Y-O distances and volumes of the YO 6 octahedra increase with increasing mean ionic radii, i.e., the Fe 3+ fi Al substitution at the Y site. However, change of the sizes of XO 6 octahedra against the mean ionic radii at the X site is not distinct, and tends to depend on the volume change of the YO 6 octahedra.Thus, the geometrical change of the YO 6 octahedra with Fe 3+ fi Al substitution at the Y site is essential for the structural changes of pumpellyite. The expansion of the YO 6 octahedra by the ionic substitution of Fe 3+ for Al causes gradual change of the octahedra to more symmetrical and regular forms.

X-ray single-crystal and optical spectroscopic study of chromian pumpellyite from Sarany, Urals, Russia

“…Both bond valence calculations and located hydrogen sites indicated that the O5, O7, O10 and O11 positions represent hydroxyl groups. The bond -valence result is consistent with previous studies Donnay, 1971, 1973;Yoshiasa and Matsumoto, 1985;Nagashima et al, 2006;.…”

“…The refined site occupancies of cations determined here are also similar to those of is not yet known. Nagashima et al (2006) and found a positive correlation between the mean Y -O distance and mean ionic radius of the Y site, but no correlation between mean X -O and the mean ionic radius of the X site. They concluded that the cation substitution at the Y site directly results in structural change.…”

X-ray Rietveld and 57Fe M^|^ouml;ssbauer study of babingtonite from Kouragahana, Shimane Peninsula, Japan

“…The hydrothermal alteration of the Kouragahana dolerite is characterized by the mineral assemblages of Ferich pumpellyite + prehnite and of prehnite + babingtonite in veins and cavities. The former assemblage indicates low -temperature alteration conditions of the zeolite to prehnite -pumpellyite facies (Kano et al, 1986;Akasaka et al, 1997;Nagashima et al, 2006). In this assemblage, prehnite occurs as an overgrowth phase on the Fe -rich pumpellyite (Kano et al, 1986;Akasaka et al, 1997), indicating later crystallization of prehnite than Fe -rich pumpellyite with decreasing fluid temperature.…”

“…In the altered dolerite at Kouragahana, Mihonosekicho, Shimane Peninsula, prehnite (Nomura et al, 1984;Akasaka et al, 2003a) and Fe -rich pumpellyite (Nagashima et al, 2006) occur in cavities or veins. Veins from a few to tens of centimeters in width and cavities with diameters of about 15 cm occur.…”

Pumpellyite-, sursassite-, and epidote-type structures: common principles-individual features

“…Moore et al (1985). (Artioli and Geiger, 1994;Akasaka et al, 1997;Artioli et al, 2003;Nagashima et al, 2006) , occupies the Y site. Cr -rich pumpellyite from Bisersk was named 'shuiskite'.…”