American Mineralogist, Volume 87, pages 739–744, 2002 0003-004X/02/0506–739$05.00 739 INTRODUCTION Hellandite is a rare borosilicate mineral, first found at Kragerö (Norway) by Brögger (1903) and studied by Brögger (1907, 1922) and by Oftedal (1964, 1965). The structure was later determined by Mellini and Merlino (1977) using a sample from Predazzo (Italy). A few other occurrences were reported over the past two decades, including the very recent descrip- tion of hellandite-(Ce) (Oberti et al. 1999 and references therein). The discovery of several non-metamict hellandite samples from the volcanic Plio-Pleistocene potassic province of Latium (the so-called Roman Comagmatic Region; Wash- ington 1906) prompted a detailed study of the crystal chemis- try of the hellandite-group minerals using a combination of * E-mail: oberti@crystal.unipv.it Ciprianiite and mottanaite-(Ce), two new minerals of the hellandite group from Latium (Italy) GIANCARLO DELLA VENTURA, 1 PAOLA BONAZZI, 2 ROBERTA OBERTI, 3, * AND LUISA OTTOLINI 3 1 Dipartimento di Scienze della Terra, Università della Calabria, I-87030 Arcavacata di Rende (CS), Italy 2 Dipartimento di Scienze della Terra, Università di Firenze, I-50121 Firenze, Italy 3 CNR-CS per la Cristallochimica e la Cristallografia (CSCC), via Ferrata 1, I-27100 Pavia, Italy 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) [ide- ally X Ca 4 Y (CeCa) Z Al T Be 2 (Si 4 B 4 O 22 ) W O 2 ] and ciprianiite [ideally X Ca 4 Y [(Th,U)(REE)] Z Al T ■ 2 (Si 4 B 4 O 22 ) 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-phos- phate 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 mag- matic 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/cm 3 , D calc is 3.88 g/cm 3 . Mottanaite- (Ce) is biaxial negative, with a = 1.680(5), b = 1.694(2), g = 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, b = 110.97(5)∞, V = 864.3(8) Å 3 for mottanaite-(Ce), and a = 19.059(5) Å, b = 4.729(1) Å, c = 10.291(4) Å, b = 111.33(2)∞, V = 864.0(5) Å 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 informa- tion obtained from the structure refinements. The crystal-chemical formulae of the crystals studied, recalculated on the basis of 24 anions, are M3,4 Ca 4 M2 [REE 1.45 Ca 0.37 (Th,U) 4 0 + .17 Y 0.01 ] S=2 M1 (Al 0.50 Fe 3 0 + .38 Mg 0.03 Ti 4 0 + .07 ) S=0.99 T (Be 1.18 Li 0.02 ■ 0.37 )B 3.99 Si 3.98 O 22 O5 (O 2 1 – .04 F 0.53 OH 0.43 ) S=2 for mottanaite-(Ce); and M3,4 Ca 4 M2 [REE 3 0 + .72 (Th,U) 4 0 + .66 Ca 0.60 Y 0.02 ] S=2 M1 (Al 0.48 Fe 3 0 + .38 Ti 4 0 + .10 Mg 0.05 Mn 3 0 + .02 ) S=1.03 T (Be 0.82 ■ 0.60 Li 0.04 ) B 4.00 Si 4.00 O 22 O5 (O 2 0 – .97 OH 0.54 F 0.49 ) S=2 for ciprianiite. microchemical (SEM-EDX, EMPA, SIMS), single-crystal X- ray diffraction, and spectroscopic (IR) methods. The collected data, together with a critical re-evaluation of the existing data on hellandites, permits the clarification of several previously unresolved problems, such as the relations between hellandite and tadzhikite, the presence of vacancies and OH groups in the structure, and a general crystal-chemical formula for these min- erals. The redefinition of the hellandite group that resulted from these studies has been approved by the International Mineral- ogical Association (IMA) commission and is published in a companion paper (Oberti et al. 2002). In this paper we report the occurrence of two new end-mem- bers of the hellandite group found in syenitic ejecta scattered throughout the pyroclastic formations of the Latium volcanic province. The new minerals and mineral names have been ap- proved by the Commission on New Minerals and Mineral