TREMBATHITE, (MG,FE)3B7O13CL A NEW BORATE MINERAL FROM THE SALT SPRINGS POTASH DEPOSIT, SUSSEX, NEW-BRUNSWICK

The new mineral species trembathite, ideally (Mg,Fe)3B7O13Cl, from the Salt Springs potash deposit, Sussex, New Brunswick, is the Mg-analogue of congolite and the rhombohedral dimorph of boracite. The mineral occurs as rhombohedra seldom exceeding 2 mm in maximum diameter. Trembathite is associated with halite, hilgardite and clay minerals. It is transparent, colorless to light blue with a vitreous luster and a white streak, it shows no cleavage, and has a conchoidal fracture and a Mohs hardness estimated at 6-8. The measured densities range from 2.84 to 3.34 g/cm 3; the calculated density is 3.14 g/cm 3. Macroscopic twinning was not observed in trembathite, but an optical examination indicates that the crystals are polysynthetically twinned. Trembathite is uniaxial negative, omega-1.684(2), epsilon-1.668(2), and is not pleochroic. It is hexagonal, space group R3c, with a 8.574(2), c 20.99(1) angstrom, V 1336.6(6) angstrom3 and Z = 6. The strongest six lines of the X-ray powder-diffraction pattern [diffractometer; d in angstrom(I)(hkl)] are: 3.497(34)(202,006), 3.028(100)(024), 2.711(66)(122,116), 2.144(37)(220,208), 2.050(73)(131,223,217,119), 1.828(25) (042,226). The electron-microprobe composition based on 0 + Cl = 14 is (Mg2.29Fe0.79 Mn0.03)SIGMA-3.11B6.89012.90Cl1.10. The name honors Lowell T. Trembath, professor at the University of New Brunswick.