MINERALOGICAL MAGAZINE, SEPTEMBER 1981, VOL. 44, PP. 269-73

Cesanite, Ca2Na3[(OH)(S04)3], a sulphate isotypic to , 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 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 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 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 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 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 , 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 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. 3.167 1012* 13 4042 1.759 12 2130* 3.092 20 3033 } 2131 2.822 60 * 1.724 20 DTA 1122* 2.784 25 ~} 2.727 100 3030* 32~2 1.648 12 2022* 2.636 49 2243 } 3031 2.535 14 3143* 1.615 9 1 * 2132* 2.302 10 4260* 1.547 11 2 3140* 2.269 47 2134* 1.509 7 3 2241 2.234 4 4261 1.506 7 4 \ * * I, 3141 2.155 12b 5052* 1.478 11 TG * t!.W% III 3032* 2.134 5b 5160 1.457 14 , TOC 1123 2.069 6 5161 1.454 18 100 200 300 400 500 600 700 800 900 1000 2023* 2.004 10 4153 1.437 11 2242* 1.948 32 FIG. 1. DT and TG traces for cesanite. Note: the dotted TG line was obtained by rapid heating. Experimental conditions: CufNi, 50 kV, 30 mA, Si internal standard 1/4 2/1 min - '. Indexing on the basis of the 9.442(4), c hexagonal celJ a = = 6.903(3). The reflections A separate analytical problem concerns the marked with asterisk are those used for refinement. volatile content. Apatite-group minerals may con- tain 0, OH, F, Cl anions in the A site (assuming usingtwelve accurately identified reflections. These the general formula X s[AI(Z04h]), whereas in the preliminary cell data were then used as starting Z site other volatile elements (particularly C) can parameters for the computation of the final cell be present. The total volatile loss of cesanite, deter- (fable I) as well as of the indices of all the observed mined by low-speed TGA followed by a continuous reflections with the self-indexing least-squares pro- heating at l000°C for 60 min., is 4.75%. However gram of Appleman and Evans (1972).The extinction this loss occurs stepwise (fig. 1): the DTA curve conditions of the P63/m space group, typical of exhibits three major endothermal peaks at 425°, apatite, were used during this calculation. 605° and 740 DC, plus a shoulder at about 760- The final cell parameters computed from the 70°C. While the first two effects involve well- powder pattern compare well with those directly defined losses (about 0.5% and 2.0% respectively) measured on single-crystals (personal communica- the third one reaches a maximum at 740°C, to tion of V. Tazzoli). continue slowly up to 1000°C (the final temperature Crystal chemistry. Electron microprobe analyses of the experiment). The volatile loss involved at were performed with a Cambridge Geoscan in- 740°C is 1.1%, bringing the total loss up to 3.60%. strument. Running conditions were: 15 kV gun The interpretation of the TGA curve is not straight- potential, 1 x 10 - 8 A specimen current, ~ 80 Jim forward. Adding the total of the microprobe analysis probe diameter, 20 sec counting time. The reference to the final loss (4.75%) or to the loss at 740°C standards were synthetic anorthite (for Ca) and (3.6%) gives an analytical sum within the limits of natural analysed orthoclase (for K),jadeite (for Na), the microprobe analyses (100.27 and 99.12 respect- and celestine (for Sr and S). This sulphate was ively). It is likely that the final TGA heat-loss may chosen as standard because the S-KIJ. peak shifts reflect volatilization of ; moreover we are considerably as a function of the sulphur coordina- aware that at 740°C and 760 °C two phase changes tion state (Chapell and White, 1968; Lovering and take place as discussed elsewhere (D. Andretta and Widdowson, 1968). No evidence of other elements S. Deganello, personal communication). (Si, P, Mg, Fe, and As) was found within the The IR spectrum of cesanite is shown in fig. 2. sensitivity of WD microprobe. Raw data were Most of the absorption bands can be referred to corrected for dead time, background, linear in- modes involving [S04]2- . However the broad strumental drift, and ZAF effects using the un- band at 3440 cm -1 is due to adsorbed water, and 272 G. CA V ARRETTA ET AL. TABLE II. Cesanite microprobe analysis, repre- possible, since the volatile (anion) content constrains senting the mean oj three determinations. Cl and F the accuracy ofthe analysis. Therefore two alterna- by colorimetry tive ways of computing the formula may be used. On a cation basis of 8 (or 16) following the analyses obtained by electron microprobe tech- Formula niques (Neumann, 1976; Stern, 1979); or on the basis of S 3 (i.e. assuming no substitution of 8 metals 5=3 = % oxide [C03J2- for [S04J2-, as suggested by chemical CaO 18.9 0.25 1.54 1.54 evidence). SrO 0.72 0.08 0.03 0.03 As far as the major elements are concerned, both Na20 23.3 0.31 3.42 3.43 procedures, when applied to cesanite, lead to K20 0.21 0.03 0.02 0.02 almost identical results, and to anionic contents S03 52.6 0.74 2.99 3.00 in good agreement with the amount required CI 0.44 0.06 0.06 to balance the formula. Given the presence of F 0.25 0.06 0.06 (Na + K) substituting for (Ca + Sr) in excess of 3, H2O 2.91 0.20 1.47 1.48 i.e. the amount required to balance the complete pS +, some H20 is needed 99.33 9.59 9.62 substitution of S6+ for o CI, F -0.21 to substitute for (OH + Cl + F) in order to fully == balance the formula. Thus, in order to account for 99.12 the newly found cesanite, the general formula for the natural apatite-group of minerals, suggested by E = maximum error for the 95% confidence level. Harada et al., 1971, as Cas(Si,S,Ph012(Cl,F,OH) should be expanded to the smooth band at 1615 to water bonding vibra- 1 (Cas xN ax)(Siy,Sx,P 3 x - y)O 12 tions. The 3570 cm - band, typical of structural - (F,Cl,OH)l_x.- (H20J. OH, is very faint and interfered with by the H20 band, Other data on the volatiles present in the This general formula will account not only for the cesanite were obtained by colorimetry (Cl, F) existence of wilkeite, ellestadite and hydroxylelle- or optical spectrography (B < 10 ppm) on single stadite (here the balanced substitution Si4+S6+ for samples. There is no C02 present in the structure. pS +pS + does not affect the Ca occupancy), but The comprehensive chemical analysis is given in also for the present coupled substitution Na+S6+ Table II. for Ca 2+pS +, which involves changes in the X Discussion. The apatite-group minerals are often group. IUs therefore suggested to restrict the name (e.g. Ramdohr and Strunz, 1978) described accord- 'sulphate-apatite' only to those minerals in the ing to the general formula X s[AI(Z04hJ where apatite group containing substantial sulphate where X = Ca, Pb, Sr (and minor amounts of REE, Ba, the main X cation is predominantly Ca (or another K, Mn2+, Fe2+, Mg, Th); A = F, Cl, OH, to; divalent cation), and to use the new name 'cesanite Z = P, As, V, S, Si, C, Cr. The formulae are group' for those sulphates, isostructural with apa- computed on the basis of 13 (or 26) anions. tite, where the monovalent cation (mostly Na) plays In the case of cesanite, such a procedure is not a dominant role in the X position.

100

II

25

4000 em 1 3000 2000 1600 1200 800 200 FIG. 2. IR spectrum of cesanite. CESANITE, A NEW MINERAL 273

Acknowledgements. Support for this investigation is due Eakle, A. S. and Rogers, A. F. (1914). Am. J. Sci. 37, 262-7. to C.N.R., M.P.I., and E.N.E.L. Thanks are due to Drs Fletcher, L. (1889). Mineral. Mag. 8, 172. L. Conforto, L. Poppi, and V. Tazzoli for halogen Funiciello, R., Locardi, E., and Parotto, M. (1976). Boll. analyses, thermal analyses (DTA and TGA), and single Soc. Geol. It. 95, 831-49. crystal work respectively. M. Albano and M. Salvati skil- - Mariotti, G., Parotto, M., Preite-Martinez, M., fully prepared the drawings. The critical advice of Pro- Tecce, F., Toneatti, R., and Turi, B. (1979). Geo- fessor Sergio Deganello and Professor Renato Funiciello thermics, 8, 55-73. iskindly appreciated. The suggestions and criticism of Dr Harada, K., Nagashima, N., Nakao, K., and Kato, A. A. Kato are also gratefully acknowledged. (1971). Am. Mineral. 56,1507-18. Klement, R. (1939). Die Naturwiss, 27, 568. REFERENCES Lovering, J. F. and Widdowson, J. R. (1968). Lithos, 1, 264- 7. Appleman, D. F. and Evans, H. T. (1972). Nat. Techn. McConnell, D. (1937). Am. Mineral. 22, 977-86. Inf. Servo U.S. Dep. Commerce, Springfield, Virginia, Maras, A. (1979). Per. Mineral. 48, 195-203. PB-216, 188. Neumann, E. R. (1976). Norsk Geol. Tidsskr. 56,1-6. Baldi, P., Civitelli, G., Funiciello, R., Lombardi, G., Ramdohr, P. and Strunz, H. (1978). KlockmannsLehrbuch Parotto, M., and Serva, L. (1976a). Intern. Congr. on der Mineralogie 16th edn., F. Enke Verlag, Stuttgart. Thermal Waters, Geothermal Energy and Volcanism of Schneider, W. (1967). Neues Jahrb. Mineral., Monatsh. the Mediterranean Area, Athens, Proc. Geoth. Energy, 284-9. I, 71-86. - (1969). Ibid. 58-64. - Funiciello, R., Locardi, E., and Parotto, M. (1976b). Stern, W. B. (1979). Schweiz. Mineral. Petrogr. Mitt. 59, Ibid. I, 43-55. 75-82. Calamai, A., Cataldi, R., Dall'Aglio, M., and Ferrara, Vasilieva, Z. V., Litzarev, M. A., and Organova, N. I. G. C. (1975). UN Symp. on Geothermal Energy, S. Fran- (1958). Dokl. Akad. Nauk SSR, 118, 577-80. (In Russian.) cisco, Cal., USA 1, 305-13. Websky, M. (1886). Sitzg-Ber. Berlin. Akad. Wiss. 1045. Chapell, B. W. and White, A. J. R. (1968). Am. Mineral. 53, 1735-8. Dihn, P. and Klement, R. (1942). Z. Elektrochem. 48, 331-3. [Manuscript received 21 January 1981]