New data on the crystal-chemistry of fluoborite by means of SREF, SIMS, and EMP analysis

The crystal structure of fluoborite [Mg3F3(BO3)] was refined by Dal Negro a nd Tadini (1974) who provided a complete structural model. Previously, Take uchi (1950) had refined an OH-dominant fluoborite (OH similar to 70%), but the limited quantity of data (extracted from two Weissenberg-Buerger photog raphs) did not permit the location of H atoms. Dal Negro and Tadini (1974) also could not locate H atoms because they used a crystal with near end-mem ber composition. We have located the H bond in an OH-dominant fluoborite fi om the Betic Cordilleras (SE Spain). Excellent quality X-ray data on two c rystals of fluoborite allowed discovery and refinement of the H position in this mineral. Electron microprobe (EMP) and secondary-ion mass spectrometr y (SIMS) analyses of the light elements H, B, and F have resulted in the fo rmulation of special procedures to obtain accurate, high quality quantitati ve data, which are presented in this paper. EMP, SIMS, and crystal structur e refinement (SREF) data are in a good agreement. Linear equations are also presented to calculate the F content directly from cell parameters.