97 macla nº 13. septiembre ‘10 revista de la sociedad española de mineralogía Fontarnauite, a New Sulphate-Borate Mineral from the Emet Borate District (Turkey) / JAVIER GARCÍA-VEIGAS (1,*), LAURA ROSELL (2), XAVIER ALCOBÉ (1), IGNACIO SUBIAS (3), FEDERICO ORTÍ (2), İBRAHIM GÜNDOĞAN (4), CAHIT HELVACI (4) (1) Serveis Cientificotècnics, Universitat de Barcelona (SCT-UB). 08028, Barcelona (España) (2) Departament de Geoquímica, Petrologia i Prospecció Geològica, Universitat de Barcelona. 08028, Barcelona (España) (3) Departamento de Ciencias de la Tierra, Universidad de Zaragoza. 50009, Zaragoza (España) (4) Dokuz Eylül Üniversitesi, Jeoloji Mühendisliği Bölümü, Tinaztepe Kampüsü. 35160, Buca-Ízmir (Turkey) INTRODUCTION. Fontarnauite is a new mineral found in core samples belonging to Doğanlar boreholes nº2 and nº188 drilled in the vicinity of the Doğanlar village, located four kilometres to the southwest of the Emet town (Western Anatolia, Turkey). Fontarnauite is a double salt (borate- sulphate) of sodium and strontium with minor contents of potassium and calcium. So far no borates of a Sr- and Na-containing sulphate group have been reported before. Despite the mm-size, single crystals have been impossible to obtain, thus hindering the appropriate characterization required for the recognition as a new mineral by the CNMNC-IMA. The proposed name is for deceased Dr. Ramon Fontarnau (1944-2007), Director of the Material Characterization Section the Scientific-Technical Survey oat the University of Barcelona. It seems a fitting and deserving honour, given his effort to promote the development of scientific facilities focused on, among others, mineral characterization. GEOLOGICAL SETTING. The western Anatolia borate deposits (Turkey) consist of five separated districs: Bigadiç, Sultançayir, Kestelet, Emet and Kirka, all of them formed during the Miocene in closed lacustrine evaporite basins during periods of high volcanic and hydrothermal activity. Although the mineral association of borates varies in each district, they are generally interbedded with series of tuffaceous deposits. The principal borate mineral outcropping in several open pit mines in the Emet basin is colemanite with minor amounts of ulexite and hydroboracite (Helvacı and Ortí, 1998). The mineralogical record of Doğanlar boreholes is characterized by the alternation of Na-Ca borate (probertite) and Na-Ca sulphate (glauberite) units including a central halite deposit. Colemanite is restricted to the base and top of the sequence. fig 1. Optical (upper) and BSE-SEM (lower) images of a cluster of fontarnauite crystals. (F: fontarnauite, P: probertite, T: tuff). Sedimentary and early diagenetic processes controlled the crystal growth both subaqueously and interstitially. Moreover bacterially inducing dolomite precipitation occurred. The composition of fluid inclusions in halite samples indicates high K concentrations in SO4- rich brines. Kalistrontite is abundant in the sequence indicating a significant concentration of Sr in brines. Fontarnauite appears as an early diagenetic phase, replacing both probertite and glauberite, as a consequence of a particular K-SO4 composition achieved by the residual brines during evaporation in a saline lake environment with volcaniclastic contribution. Other minor minerals found in the boreholes include: borates (aristarainite, colemanite, hydroboracite, kaliborite tunellite and ulexite), sulphates (anhydrite, celestine, gypsum, kalistrontite and thenardite), and sulphides (arsenopyrite, orpiment and realgar). PHYSICAL AND OPTICAL PROPERTIES. Fontarnauite occurs as colourless or slightly brown isolated crystals, or as crystal clusters (Fig. 1), with prismatic sections less than 5 mm long. More often it displays pseudohexagonal sections less than 1 mm in diameter. Under electron microscope it shows a perfect cleavage parallel to (010) (Fig. 2). It has a pearly, transparent to translucent lustre, a brittle tenacity and a white streak. fig 2. BSE-SEM image of fontarnauite showing a perfect cleavage and replacing a fine-grained probertite. Optically, it is biaxial negative and non- pleochroic. It is colourless and displays low positive relief in thin section. Interference colours in prismatic sections are first-order grey and white (Fig. 3). Centred acute bisectrix figures with numerous isochromes are obtained on sections parallel to (010). Optic axis dispersion is medium to weak (r > v) and crossed bisectrix dispersion is distinct. palabras clave: boratos, evaporitas, Mioceno. key words: borates, evaporites, Miocene. resumen SEM 2010 * corresponding author: garcia_veigas@ub.edu