Crystal-chemistry of a complex Mn-bearing alkali amphibole ("tirodite") on the verge of exsolution

Abstract

Electron- and ion-microprobe and Mossbauer analyses combined with X-ray structure refinement of a straw-yellow Mn-bearing alkali amphibole from Tirodi, Maharashtra, India, with no evidence of exsolution or inclusions, indicate a composition (Na 0.30 K 0.03 )(Na 0.87 Ca 0.39 Mn 0.57 Mg 0.17 )(Mg 3.84 Fe 3+ 0.72 Mn 0.38 Li 0.06 )(Si 7.88 Al 0.12 ) O 22 F 0.40 (OH) 1.60 , nearly half-way between (Ca-Na) and (Fe-Mg-Mn) amphiboles. The unit-cell dimensions are: a = 9.704 (7), b = 17.990 (12), c = 5.297 (3) A, β = 103.51 (5)°; space group C2/m and Z = 2. The single-crystal X-ray structure refinement (1363 reflections, R all = 0.039) was done to characterize the crystal-chemical features that might act as indicators of incipient exsolution phenomena due to immiscibility between (Ca-Na) and (Fe-Mg-Mn) amphibole compositions. An 57 Fe Mossbauer spectrum shows all iron to be in the trivalent state. All manganese is in the divalent state, and preferentially occurs at the split M4' site. The value of the β angle is related to the cation content at M4, and is consistent with the intermediate composition. C Mn 2+ is distributed almost equally over the three octahedral sites M1, M2, M3. A small amount of Li (detected by ion-microprobe) is ordered at the M3 site with concomitant substitution of Fe 3+ at the M2 site for charge balance