The oxo-component (WO2-) in amphiboles is crucial both for mineralogy (it is the first discriminating parameter in the new and crystal-chemistry-based scheme for amphibole classification, Hawthorne et al., 2012) and petrology (it is a clue for fO2 conditions of crystallization). Also, oxo-amphiboles are far more common than previously thought. WO2- can be stabilized by two crystal-chemical mechanisms, which both occur mainly at the M(1) site and secondly at the M(3) site: (1) the oxidation of Fe2+, and (2) the presence of Ti4+ or Mn3+. Whereas the first mechanism is typical of magmatic systems, and can be simulated and studied in operando in the lab, the second mechanism implies peculiar bulk compositions and fO2 conditions of crystallization. This is the case for the two new end-members introduced in this work: "oxo-mangani-leakeite", ANaBNa2C(Mn3+4Li) TSi8 O22 WO2, and "oxo-potassic-richterite", AKB(NaCa)C(Mg4Ti) TSi8 O22 WO2 (inverted commas are due because approval by IMA-CNMNC is still underway). "Oxo-mangani-leakeite" occurs together with mangano-mangani-ungarettiite, ANaBNa2C(Mn2+2Mn3+3)TSi8 O22WO2, in strongly foliated oxidized assemblages in the Hoskins' Mn mine (Australia). However, these two amphiboles never occur in the same sample, because the first requires both the presence of M(3)Li and complete oxidation of Mn at M(1,2), whereas the second requires significant Mn2+ and has an inverse ordering scheme for C cations: C2+ at M(2), C3+ at M(1,3). Solid-solution between the two end-member compositions is therefore crystal-chemically not possible. Accordingly, we have characterized "oxo-mangani-leakeite" compositions with 0.65 < CLi <0.50 apfu but could only find nearly stoichiometric mangano-mangani-ungarettiite compositions. "Oxo-potassic-richterite" was found in a lava dome at Leucite Hill, Wyoming, a locality known for potassic-fluoro-richterite. The rock sample contains acicular amphibole crystals, with a strong zoning in Ti, where lower Ti contents correspond to higher WF- and lower WO2- contents. This unusual solid-solution must be related to variations in the composition of the circulating fluids. It is noteworthy that amphiboles with richteritic compositions rarely contain high-charged cations other than Ti (in lamproites), and this Ti occurs as both a C and a T cation (they usually have low WO2- contents; Oberti et al. 1992). Even more interesting, synthetic oxo amphiboles with compositions similar to "oxo-potassic-richterite" were obtained many years ago (Tiepolo, unpublished) while trying to model trace-element partitioning in high-Si Ti-enriched lamproites. Hawthorne F.C., Oberti R., Harlow G.E., Maresch W.V., Martin R.F., Schumacher J.C. & Welch M.D. 2012. Nomenclature of the amphibole supergroup. Am. Mineral., 97, 2031-2048. Oberti R., Ungaretti L., Cannillo E., Hawthorne F.C. 1992. The behaviour of Ti in amphiboles: I. Four- and six-coordinated Ti in richterites. Eur. J. Mineral., 4, 425-439.
Impossible or unusual solid-solutions in oxo-amphiboles: mangano-mangani-ungarettiite vs. "oxo-mangani-leakeite" and potassic-fluoro-richterite vs. "oxo-potassic-richterite"
Oberti R;
2015
Abstract
The oxo-component (WO2-) in amphiboles is crucial both for mineralogy (it is the first discriminating parameter in the new and crystal-chemistry-based scheme for amphibole classification, Hawthorne et al., 2012) and petrology (it is a clue for fO2 conditions of crystallization). Also, oxo-amphiboles are far more common than previously thought. WO2- can be stabilized by two crystal-chemical mechanisms, which both occur mainly at the M(1) site and secondly at the M(3) site: (1) the oxidation of Fe2+, and (2) the presence of Ti4+ or Mn3+. Whereas the first mechanism is typical of magmatic systems, and can be simulated and studied in operando in the lab, the second mechanism implies peculiar bulk compositions and fO2 conditions of crystallization. This is the case for the two new end-members introduced in this work: "oxo-mangani-leakeite", ANaBNa2C(Mn3+4Li) TSi8 O22 WO2, and "oxo-potassic-richterite", AKB(NaCa)C(Mg4Ti) TSi8 O22 WO2 (inverted commas are due because approval by IMA-CNMNC is still underway). "Oxo-mangani-leakeite" occurs together with mangano-mangani-ungarettiite, ANaBNa2C(Mn2+2Mn3+3)TSi8 O22WO2, in strongly foliated oxidized assemblages in the Hoskins' Mn mine (Australia). However, these two amphiboles never occur in the same sample, because the first requires both the presence of M(3)Li and complete oxidation of Mn at M(1,2), whereas the second requires significant Mn2+ and has an inverse ordering scheme for C cations: C2+ at M(2), C3+ at M(1,3). Solid-solution between the two end-member compositions is therefore crystal-chemically not possible. Accordingly, we have characterized "oxo-mangani-leakeite" compositions with 0.65 < CLi <0.50 apfu but could only find nearly stoichiometric mangano-mangani-ungarettiite compositions. "Oxo-potassic-richterite" was found in a lava dome at Leucite Hill, Wyoming, a locality known for potassic-fluoro-richterite. The rock sample contains acicular amphibole crystals, with a strong zoning in Ti, where lower Ti contents correspond to higher WF- and lower WO2- contents. This unusual solid-solution must be related to variations in the composition of the circulating fluids. It is noteworthy that amphiboles with richteritic compositions rarely contain high-charged cations other than Ti (in lamproites), and this Ti occurs as both a C and a T cation (they usually have low WO2- contents; Oberti et al. 1992). Even more interesting, synthetic oxo amphiboles with compositions similar to "oxo-potassic-richterite" were obtained many years ago (Tiepolo, unpublished) while trying to model trace-element partitioning in high-Si Ti-enriched lamproites. Hawthorne F.C., Oberti R., Harlow G.E., Maresch W.V., Martin R.F., Schumacher J.C. & Welch M.D. 2012. Nomenclature of the amphibole supergroup. Am. Mineral., 97, 2031-2048. Oberti R., Ungaretti L., Cannillo E., Hawthorne F.C. 1992. The behaviour of Ti in amphiboles: I. Four- and six-coordinated Ti in richterites. Eur. J. Mineral., 4, 425-439.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
