MINERALOGICAL MAGAZINE, SEPTEMBER 1981, VOL. 44, PP. 269-73 Cesanite, Ca2Na3[(OH)(S04)3], a sulphate isotypic to apatite, from the Cesano geothermal field (Latium, Italy) G. CAVARRETTA Centro Di Studio per la Geologia dell'Italia Centrale del CN.R., c/o Istituto di Geologia e Paleontologia, Citta Universitaria, 00185 Roma, Italy A. MOTTANA Cattedra di Mineralogia, Istituto di Mineralogia e Petrografia, Ciua Universitaria, 00185 Roma, Italy AND F. TECCE Progetto Finalizzato 'Energetica' del CN.R., c/o Istituto di Geologia e Paleontologia, CiUa Universitaria, 00185 Roma, Italy ABSTRACT. Cesanite occurs both as a solid vein (1 cm Common occurrences are gypsum and an- thick) and as a cavity-filling of an explosive breccia in hydrite, although unusual minerals like glaserite core samples of the Cesano 1 geothermal well (Cesano K3Na[S04J2, gorgeyite K2CaS[S04J6' H20 (Funi- area, Latium, Italy). cielloet al., 1979),kalistrontite K2Sr[S04J2 (Maras, Cesanite is colourless, medium to coarse grained, soft (H g 1979), and glauberite Na2Ca[S04J2' have also = 2 to 3) and light (Pmoas 2.786:t 0.002 cm - 3). It is uniaxial negative, E 1.564, W 1.570, with space group been detected in core samples. = = Another unusual mineral, initially misidentified P63/m and cell parameters a = 9.442 (4), c = 6.903 (3) A, for cia = 0.730. Identifying spacings are 8.161, 2.822, as a phosphate on the basis of X-ray powder 2.727,1.844 A, in X-ray powder patterns strikingly similar diffraction analyses, has been repeatedly detected to those of apatite. The chemical formula (microprobe in several cores. Eventually, a solid vein (1 cm thick) analyses on two grains) is Ca1.53Sro.o3Na3.42Ko.o2[(Clo.o6 in a core sample at 1252.10-1254.10 m below . F O.060HO.44)(S2.990 dJ 0.44H20, while the theoretical surface in the Cesano 1 well was found. Although formula, derived from considerations on structural identity the X-ray powder pattern was very similar to that with apatite, is Ca2Na3[(OH)(S04hJ. Cesanite is the end member of the apatite-wilkeite-ellestadite series where of apatite, the mineral was eventually recognized [P04J3- is entirely substituted by [S04J2-, the charge to be a sulphate on the basis of a SEM-EDS balance being made up by Na + substituting for Ca2+. analysis. Detailed studies showed it to be a new species, with a crystal structure isotypic with that THE Cesano geothermal field is located east of of apatite (V. Tazzoli, private communication). Lake Bracciano, north of Rome (Latium, Italy), Apatite-like sulphates are not new in the mineral- where volcanic and pyroclastic rocks related to the ogical record. Caracolite Na3Pb2[Cll(S04hJ, de- Pleistocene activity of the Sabatini volcanic system scribed by Websky in 1886, was recognized to be occur. The field is currently being drilled by ENEL, isotypic with apatite by Fletcher as early as 1889 the Italian National Electricity Board, because of (see Schneider, 1967, 1969). Apatite itself shows its rather high heat-flow anomaly combined with wide substitutions of the [P04J3 - group by equal a structural high of the carbonatic basement under amounts of [Si04J4- and [S04J2- leading to a cover of mainly pyroclastic material (Baldi et al., ellestadite Cas[(F,Cl,OH)I(Si04,S04)3J (McCon- 1976a,b; Funiciello et al., 1976, 1979). So far, nell, 1937),and hydroxylellestadite (Harada et al., exploitation has proved to be difficult because of 1971), through to the intermediate variety wilkeite the technical problems related to the highly con- Cas[(F,O,Cl,OH)I(S04,Si04,P04)3J (Eakle and centrated brines (Calamai et al., 1975). Rogers, 1914). @ Copyright the Mineralogical Society 270 G. CA VARRETTA ET AL. However, this is the first report of a Ca-Na- cesanite crystals are smaller, and are intergroWD sulphate, the fluorine-bearing synthetic equivalent with g6rgeyite. of which has been known for a long time (Klement, Physical properties. Cesanite is white massivebut 1939; Dihn and Klement, 1942). A reported natural colourless in individual grains. Crystals are trans- occurrence of similar material, named sulphate- parent to translucent, mostly subhedral, short apatite (Vasilieva et al., 1958) lacks comprehensive prismatic, hexagonal shaped, usually with no ter. analytical data. minal faces and an elongation to diameter ratio The new mineral has been named cesanite after between 2: 1 and 4: 1. They show a pronounced the locality of occurrence. The description of the cleavage normal to the elongation and striations species and proposed name have been approved on the prism faces. Single crystals show greasy by the Commission on New Minerals and Mineral lustre, while agglomerates occasionally appear quite Names of the IMA. Type material is deposited in silky. Careful and gentle pressure to separate the Museum of Mineralogy of the University of individual grains enabled a few euhedral crystals Rome (specimen no. 24316/1). Material from the to be isolated. The prisms {lOTO}display a slight same vein has been also presented to the Natur- flattening according to two parallel faces, and, historisches Museum Wien (Austria) and to the correspondingly, an upper pyramid {IOIl} with City Museum of Natural History, Milan (Italy). two faces better developed than the others. They Occurrence. Cesano 1 geothermal well is located are truncated by a pinacoid {0001} or by another 2.3 km north of the village of Cesano, right on the unevenly developed pyramid {lOIl}, simulating a edge of the Baccano caldera. We here defer from sphenoidal end. Occasionally twins according to a detailed discussion of the structural geology, (IOTa) were also detected. volcanology, stratigraphy, as well as the mineral- Hardness was measured by scratching the crystals ogy of the Cesano wells since they have been (Mohs' scale: 2-3). Density, measured by floating reported elsewhere (Baldi et al., 1976a, b; Funiciello two small crystals in a mixture of acetone and et al., 1976, 1979). It is just stressed that the Cesano methylene iodide, is 2.786:t 0.002 g em - 3. A similar 1 well was drilled to a depth of 1430 m. A volcanic measurement, carried out using diluted Clerici sequence (mainly phreatomagmatic pyroclastics solution, gave densities ranging between 2.96 and with few interbedded lava flows) extends to 1070 m, 3.02 g cm-3, thus suggesting that some reaction and is followed by a flyschoid allochtonous complex could occur (i.e. adsorption of Tl). (shales, marls, sandstones, and limestones: Middle- The solubility of cesanite in water is low, although Cretaceous to Miocene) down to 1390 m. Marls etch-pits develop after prolonged contact with and marly limestones with cherts and flints, sup- distilled water. With HCl and H2S04, both cold posed to be the upper part of the Tuscan Nappe, and hot, the grains show rounded edges; however, the so-called 'Carbonatic Basement' conclude the on the whole, they remain clear. On the contrary, drilled sequence. reactions with hot HN03 not only produce cor- Hydrothermal minerals have been found in all roded edges, but also a superficial film of white the horizons: the most frequent are sulphates and powdery material. Cesanite crystals are entirely calcite; occasionally ankerite, dolomite, K-feldspar, dissolved in aqua regia after a short time. magnesian calcite, sulphides, and zeolites could be In thin section cesanite is colourless, non- detected. pleochroic, moderately birefringent, uniaxial with The vein, which yielded cesanite, g6rgeyite, and negative elongation and optic sign. Refractive in- pyrite, occurs within the marls of the allochthonous dices, measured in white light using the method of complex and it is clearly related to the self-sealing the colour fringes of the Becke line on crystals process of the fractures. Such a vein is at about oriented on the universal stage, are w = 1.570(2) 45° from the vertical defined by the walls of the and t; = 1.564(2). core; the stabilized temperature value at this depth X-ray data. Powder diffraction data of cesanite is 190°C. are listed in Table I. They have been obtained by The vein is white, with sugary texture and grain low speed diffractometry (1/4°/min) both on pure size ranging from 0.2 to 1.5 mm. Individual crystals, and Si-added smear mounts. The intensities reported mostly subhedral and loosely packed, can be easily in Table I were directly measured on diffractometer removed. In general they do not show preferred traces and may reflect orientation effects due to orientation but, rather, a kind of decussate texture the well-developed cleavage. with rational grain boundaries. Occasionally small The X-ray pattern is very similar to those of vugs can be seen, where individual crystals show apatite and wilkeite; consequently it was indexed development of the upper terminal faces. Some by comparison with the indices reported in ASTM vugs are coated by a thin crust of brownish-grey cards (nos. 9-432, 6-454) for these two minerals. fine-grained gypsum. Close to the walls of the vein, Initial unit-cell parameters were then calculated CESANlTE, A NEW MINERAL 271 TABLEI. X -ray powder diffraction data of cesan- published program ELEXA, coded by one of the ite authors (G.c.) for the UN IVAC 1100/20 computer. Under the electron beam, even when this is hkli hkli strongly defocussed as in the present case, cesanite readily decomposes making repeated spot analyses 1010* 8.161 8 3142* 1.895 14 very difficult. Although a slight zonation (core to 1011* 5.263 5 3250* 1.877 4 rim: CaO+0.55%, Na20-0.31%) was detected, 1120* 4.720 46 2113* 1.844 71 the large spot size did not allow a careful investi- 1121 3.896 66 3251* 1.811 21 * gation; thus only average values are represented 0002* 3.448 87 4150* 1.785 13 here.