Vicanite-(Ce): A new Ca-Th-REE borosilicate from the Vico volcanic district (Latium, Italy)
Maras, Adriana; Parodi, Gian Carlo; Della Ventura, Giancarlo; Ohnenstetter, Daniel
European Journal of Mineralogy Volume 7 Number 2 (1995), p. 439 - 446
published: Mar 29, 1995
manuscript accepted: Oct 12, 1994
manuscript received: Aug 15, 1992
ArtNo. ESP147050702018, Price: 29.00 €
Abstract Vicanite-(Ce), ideally (Ca, REE, Th) 15As5+ (As3+0.5Na0.5)Fe3+ Si6 B4O40 F7 was found inside a volcanic ejectum collected from a pyroclastic formation of the Vico volcanic complex at Tre Croci, Vetralla (north of Rome), Viterbo province, Italy. The host rock is composed of K-feldspar (high sanidine), plagioclase, clinopyroxene plus minor quantities of magnetite and clinoamphibole. Vicanite-(Ce) occurs in small myarolitic cavities in association with zircon, thorite, thorian uraninite, betafite, thorian hellandite, titanite, antimonian asbecasite, apatite, stillwellite-(Ce), and other unidentified phases. It is found as rare euhedral crystals up to 0.3 mm across, yellowish green in colour; uniaxial (-), 8 1.722(2), ω 1.757(2), non pleochroic. The streak is white, and the mineral shows no fluorescence under UV radiation. The crystal chemical formula calculated from electron microprobe analyses, on the basis of 47(O+F) and B fixed to 4 atoms, is: (Ca8.03 Ce2.32 La1.95 Th1.82 P0.44 Nd0.28 U0.19)Σl5.06 ( As5+0 .86 P0.14)Σ1.00 (As3+0.34 Na0.12)Σ0.46 (Fe3+0.57Ti0.05Al0.08)Σ0.67 Si6.07B4 (O36.57 F10.43)Σ47. Density is higher than 4.2. The mineral is trigonal and crystallizes in the non-centrosymmetric R3m space group. The refined unit-cell parameters are: a = 10.795(1), and c = 27.336(4) Å, Z = 3 (hexagonal cell). The eight strongest X-ray diffraction lines are [d in Å (I) (hkl)]: 7.70 (50) (012), 4.42 (50) (202), 3.13 (50) (214), 2.993 (100) (027), 2.950 (70) (303), 2.698 (50) (220), 1.839 (50) (3,0,12), 1.802 (50) (2,0,14). The main feature of the structure is the presence of a new polyanion (Si3 B3 O18)15-. The cell dimensions of vicanite-(Ce) are very close to those of okanoganite-(Y), but the chemical composition is rather different as no Y, Gd, Dy, Sm and Er are present in vicanite-(Ce). The name is for the Vican volcanic complex where the mineral was found. Holotype is deposited in Museo di Mineralogia, Università di Roma "La Sapienza" (MMUR).