Two new minerals of the hellandite group were found within alkali-syenitic ejecta enclosed in pyroclastic formations of the Roman Comagmatic Province (Latium, Italy). Mottanaite-(Ce) [ideally,Ca-X(4)Y(CeCu)Al-Z(T) Be-2(Si4B4O22)O-W(2)] and ciprianiite [ideally Ca-X(4)Y[(Th,U)(REE)](Z)Al(T)square(2)(Si4B4O22) (W)(OH, F)(2)] occur as transparent, brown-colored, tabular euhedral crystals in miarolitic cavities and voids of the ejecta, which consist mainly of sanidine and plagioclase (An ranging from 20 to 80%), with minor amounts of feldspathoid, clinopyroxene and/or clinoamphibole, magnetite, titanite, and zircon. Locally, accessory minerals include britholite-(Ce), baddeleyite, phosphate to silico-phosphate phases close in composition to the brabantite-cheralite series, thorite, fluorite, danburite, and vonsenite. The genesis of the new hellandite end-members can be related to late-stage post magmatic hydrothermal fluids enriched in Zr, Ti, REEs, and actinide elements. Both mottanaite-(Ce) and ciprianiite have a vitreous luster and are non-fluorescent. Cleavage is absent in mottanaite-(Ce), fair to good in ciprianiite, {100}. Twinning is frequently observed in ciprianiite. Due to the strong intra-crystalline chemical zoning and twinning, physical properties could be measured only for mottanaite-(Ce). D-meats is 3.61(4) g/cm(3), D-calc is 3.88 g/cm(3). Mottanaite-(Ce) is biaxial negative, with alpha = 1.680(5), beta = 1.694(2), gamma = 1.708(5); 2V(meas) similar to90degrees. Both minerals are monoclinic, space group P2/a, Z = 2. Unit-cell parameters for the crystals Studied are: a = 19.032(9) Angstrom, b = 4.746(3) Angstrom, c = 10.248(5) Angstrom, beta = 110.97(5)degrees, V = 864.3(8) Angstrom(3) for mottanaite-(Ce), and a = 19.059(5) Angstrom, b = 4.729(1) Angstrom, c = 10.291(4) Angstrom, beta = 111.33(2)degrees, V = 864.0(5) Angstrom(3) for ciprianiite. Single-crystal structure refinement confirmed the presence of a further distorted tetrahedral site which was first detected in a hellandite-(Ce) sample from Latium (Oberti et al. 1999). This site is occupied by Be (+/-Li) in stoichiometric mottanaite-(REE), whereas it mainly hosts hydrogen (bonded to the O5 oxygen atom) in ciprianiite and hellandite-(REE); solid solution between the end-members is possible, as shown by the studied samples. The chemical composition of the refined crystals was obtained by combining EMPA (for medium-Z elements) and SIMS analyses (for low- and high-Z elements); their results are in excellent agreement with the chemical information obtained from the structure refinements, The crystal-chemical formulae of the crystals studied, recalculated on the basis of 24 anions, are Ca-M3.4(4)M2[REE1.45Ca0.37(Th,U)(0.17)Y-4+(0.01)](Sigma=2)(M1)(Al-0.50 Fe0.383+Mg0.03Ti0.074+)(Sigma=0.99)(T)(Bc(1.18)Li(0.02)square(0.37))B3.99Si3.98O220.5(O1.042-F0.53OH0.43)(Sigma=2) for mottanaite-(Ce); and Ca-M3.4(4)M2[REE0.723+(Th,U)(0.66)4+Ca0.60Y0.02](Sigma=2)(M1)(Al0.48Fe0.383+Ti0.104+Mg0.05Mn0.023+)(Sigma=1.03)(T)(Be(0.82)square(0.60)Li(0.04)) B4.00Si4.00O22O5((O0.97OH0.54F0.49)-O-2)(Sigma=2) for ciprianiite.
DELLA VENTURA, G., Bonazzi, P., Oberti, R., Ottolini, L. (2002). Ciprianiite and mottanaite-(Ce), two new minerals of the hellandite group from Latium (Italy). AMERICAN MINERALOGIST, 87, 739-744.
Ciprianiite and mottanaite-(Ce), two new minerals of the hellandite group from Latium (Italy)
DELLA VENTURA, Giancarlo;
2002-01-01
Abstract
Two new minerals of the hellandite group were found within alkali-syenitic ejecta enclosed in pyroclastic formations of the Roman Comagmatic Province (Latium, Italy). Mottanaite-(Ce) [ideally,Ca-X(4)Y(CeCu)Al-Z(T) Be-2(Si4B4O22)O-W(2)] and ciprianiite [ideally Ca-X(4)Y[(Th,U)(REE)](Z)Al(T)square(2)(Si4B4O22) (W)(OH, F)(2)] occur as transparent, brown-colored, tabular euhedral crystals in miarolitic cavities and voids of the ejecta, which consist mainly of sanidine and plagioclase (An ranging from 20 to 80%), with minor amounts of feldspathoid, clinopyroxene and/or clinoamphibole, magnetite, titanite, and zircon. Locally, accessory minerals include britholite-(Ce), baddeleyite, phosphate to silico-phosphate phases close in composition to the brabantite-cheralite series, thorite, fluorite, danburite, and vonsenite. The genesis of the new hellandite end-members can be related to late-stage post magmatic hydrothermal fluids enriched in Zr, Ti, REEs, and actinide elements. Both mottanaite-(Ce) and ciprianiite have a vitreous luster and are non-fluorescent. Cleavage is absent in mottanaite-(Ce), fair to good in ciprianiite, {100}. Twinning is frequently observed in ciprianiite. Due to the strong intra-crystalline chemical zoning and twinning, physical properties could be measured only for mottanaite-(Ce). D-meats is 3.61(4) g/cm(3), D-calc is 3.88 g/cm(3). Mottanaite-(Ce) is biaxial negative, with alpha = 1.680(5), beta = 1.694(2), gamma = 1.708(5); 2V(meas) similar to90degrees. Both minerals are monoclinic, space group P2/a, Z = 2. Unit-cell parameters for the crystals Studied are: a = 19.032(9) Angstrom, b = 4.746(3) Angstrom, c = 10.248(5) Angstrom, beta = 110.97(5)degrees, V = 864.3(8) Angstrom(3) for mottanaite-(Ce), and a = 19.059(5) Angstrom, b = 4.729(1) Angstrom, c = 10.291(4) Angstrom, beta = 111.33(2)degrees, V = 864.0(5) Angstrom(3) for ciprianiite. Single-crystal structure refinement confirmed the presence of a further distorted tetrahedral site which was first detected in a hellandite-(Ce) sample from Latium (Oberti et al. 1999). This site is occupied by Be (+/-Li) in stoichiometric mottanaite-(REE), whereas it mainly hosts hydrogen (bonded to the O5 oxygen atom) in ciprianiite and hellandite-(REE); solid solution between the end-members is possible, as shown by the studied samples. The chemical composition of the refined crystals was obtained by combining EMPA (for medium-Z elements) and SIMS analyses (for low- and high-Z elements); their results are in excellent agreement with the chemical information obtained from the structure refinements, The crystal-chemical formulae of the crystals studied, recalculated on the basis of 24 anions, are Ca-M3.4(4)M2[REE1.45Ca0.37(Th,U)(0.17)Y-4+(0.01)](Sigma=2)(M1)(Al-0.50 Fe0.383+Mg0.03Ti0.074+)(Sigma=0.99)(T)(Bc(1.18)Li(0.02)square(0.37))B3.99Si3.98O220.5(O1.042-F0.53OH0.43)(Sigma=2) for mottanaite-(Ce); and Ca-M3.4(4)M2[REE0.723+(Th,U)(0.66)4+Ca0.60Y0.02](Sigma=2)(M1)(Al0.48Fe0.383+Ti0.104+Mg0.05Mn0.023+)(Sigma=1.03)(T)(Be(0.82)square(0.60)Li(0.04)) B4.00Si4.00O22O5((O0.97OH0.54F0.49)-O-2)(Sigma=2) for ciprianiite.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
