Ciprianiite and mottanaite-(Ce), two new minerals of the hellandite group from Latium (Italy)

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 ^XCa^Y₄ (CeCa)^ZAl^TBe₂(Si₄B₄O₂₂)^WO₂] and ciprianiite [ideally ^XCa^Y₄ [(Th, U)(REE)]^ZAl^T□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_meas is 3.61(4) g/cm³, D_calc is 3.88 g/cm³. Mottanaite-(Ce) is biaxial negative, with α = 1.680(5), β = 1.694(2), γ = 1.708(5); 2V_meas ~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) ų 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) ų 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 ^M3,4Ca₄^M2[REE_1.45Ca_0.37(Th, U)⁴⁺_0.17Y_0.01]_∑=2^M1(Al_0.50Fe³⁺_0.38Mg_0.03Ti⁴⁺_0.07)_∑=0.99^T(Be_1.18Li_0.02□_0.37)B_3.99Si_3.98O₂₂^O5(O²⁻_1.04F_0.53OH_0.43)_∑=2 for mottanaite-(Ce); and ^M3,4Ca₄^M2[REE³⁺_0.72(Th, U)⁴⁺_0.66Ca_0.60Y_0.02]_∑=2^M1 (Al_0.48Fe³⁺_0.38Ti⁴⁺_0.10Mg_0.05Mn³⁺_0.02)_∑=1.03^T (Be_0.82□_0.60Li_0.04)B_4.00Si_4.00O₂₂^O5 (O²⁻_0.97OH_0.54F_0.49)_∑=2 for ciprianiite.