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 XCaY4 (CeCa)ZAlTBe2(Si4B4O22)WO2] and ciprianiite [ideally XCaY4 [(Th, U)(REE)]ZAlT2(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). Dmeas is 3.61(4) g/cm3, Dcalc is 3.88 g/cm3. Mottanaite-(Ce) is biaxial negative, with α = 1.680(5), β = 1.694(2), γ = 1.708(5); 2Vmeas ~90°.

Both minerals are monoclinic, space group P2/a, Z = 2. Unit-cell parameters for the crystals studied are: a = 19.032(9) Å, b = 4.746(3) Å, c = 10.248(5) Å, β = 110.97(5)°, V = 864.3(8) Å3 for mottanaite-(Ce), and a = 19.059(5) Å, b = 4.729(1) Å, c = 10.291(4) Å, β= 111.33(2)°, V = 864.0(5) Å3 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 M3,4Ca4M2[REE1.45Ca0.37(Th, U)4+0.17Y0.01]∑=2M1(Al0.50Fe3+0.38Mg0.03Ti4+0.07)∑=0.99T(Be1.18Li0.020.37)B3.99Si3.98O22O5(O2−1.04F0.53OH0.43)∑=2 for mottanaite-(Ce); and M3,4Ca4M2[REE3+0.72(Th, U)4+0.66Ca0.60Y0.02]∑=2M1 (Al0.48Fe3+0.38Ti4+0.10Mg0.05Mn3+0.02)∑=1.03T (Be0.820.60Li0.04)B4.00Si4.00O22O5 (O2−0.97OH0.54F0.49)∑=2 for ciprianiite.

You do not currently have access to this article.