The mineralogy and crystal chemistry of manganiferous tourmalines: Limits of cation substitution in synthetic and natural species

Experiments in the Na-Mg-Mn-Al-Si-B-O-H system at 375-{dollar}700spcirc{dollar}C and 2 kbar reveal distinct regions in T-X{dollar}sb{lcub}rm Mn{rcub}{dollar} space where tourmaline (Tur A and B), garnet (spessartine), and a two-phase mixture of tourmaline and garnet occur. As X{dollar}sb{lcub}rm Mn{rcub}{dollar} = Mn/(Mg+Mn) increases, the amount of tourmaline in the run products decreases, relative to Mn-rich garnet or to an equivalent low-temperature garnet break-down product. Rhodochrosite or Mn-chlorite may replace garnet below {dollar}600spcirc{dollar}C for some intermediate compositions, and appear generally at lower temperatures when the composition is Mn-rich. The lower boundary of the two-phase field (Tur B + Grt) slopes upward with increasing Mg content, consistent with a decrease in garnet stability (and a concomitant increase in tourmaline stability) as the pyrope component increases at this relatively low pressure. Specific phase assemblages that appear in the syntheses runs at intermediate compositions and temperatures vary with time: with longer runs, the two-phase field for Tur B + Grt expands.; The tourmaline from the two-phase field (Tur B) is more Mn-rich than Tur A from the single phase field. These two varieties of tourmaline also differ in the order states, for cations in the Y- and Z-sites in the structure--Tur A has S values between 0.3 and 0.5 while Tur B has S = 0.65 to 0.75. As a result of these differences in ordering, the cell parameters and structural distortions are different in the tourmaline types as well.; In experiments at 2 kbar in the Na-Mn-Al-Si-B-O-H system (X{dollar}sb{lcub}rm Mn{rcub} = 1.0),{dollar} the Mn-rich bulk chemistry promotes garnet crystallization relative, to tourmaline. Tur C appears only in a narrow temperature range between {dollar}425spcirc{dollar} and {dollar}500spcirc{dollar}C. At higher temperatures tourmaline is absent, and a "single"-phase field for Sps ({dollar}pm{dollar} high Ab {dollar}pm{dollar} Crd + Glass) is present. Reversal experiments at X{dollar}sb{lcub}rm Mn{rcub} = 1{dollar} fix the boundary between the Tur C and Sps between {dollar}500spcirc{dollar} and {dollar}600spcirc{dollar}C. Tur C shows extreme values of structural distortion, with the maximum amount of Mn in the structure of {dollar}{lcub}sim{rcub}1.6{dollar} apfu.; A study of manganiferous tourmalines from a rare-metal pegmatite near the village of Latinka, Rhodope Mountains, Bulgaria, and some other localities reveals multiple-stage compositional evolution: schorl {dollar}Rightarrow{dollar} Mn-schorl {dollar}Rightarrow{dollar} Mn-Fe-elbaite {dollar}Rightarrow{dollar} Al-elbaite. Spessartine garnet has a higher affinity for Mn than tourmaline (which has a higher affinity for Al), creating conditions for selective element distribution in rare-metal pegmatites. The fact that Sps (or Rds) was stable during the evolutionary stage of formation of this pegmatite within a mildly acidic environment indicates that tourmaline will never be much more manganiferous than allowed by the bulk Mn:Al ratio 1.5:7.5 and the following general formula:{dollar}{dollar}rm Na(Mnsb{lcub}1.5{rcub}Alsb{lcub}1.5{rcub})Alsb6(Sisb6Osb{lcub}18{rcub})(BO sb3)sb3Osb{lcub}1.5{rcub}(OH)sb{lcub}2.5{rcub}{dollar}{dollar}; The results from this project are the first pointing toward the presence of structural controls on the cation substitutions in tourmaline. The presence of extreme structural distortions in the most manganiferous synthetic tourmalines, and the evidence from natural Tur - Sps paragenesis, demonstrate that the hypothetical formula for tsilaisite with a bulk Mn:Al ratio of 3:6 cannot be obtained at temperatures in the range 375-{dollar}700spcirc{dollar}C and 2 kbar pressures.