VARENNESITE, A NEW SPECIES OF HYDRATED NA-MN SILICATE WITH A UNIQUE MONOPHYLLOSILICATE STRUCTURE

Varennesite, a silicate of Na and Mn, is a new mineral species from th e Demix-Varennes quarry, Vercheres County, Quebec. It occurs as pale b rownish yellow to orange, tabular crystals up to 4 mm in length. Assoc iated minerals commonly include eudialyte, aegirine, natrolite, serand ite, mangan-neptunite, microcline, albite and zakharovite, and rarely astrophyllite, villiaumite, makatite and vuonnemite. It is brittle, wi th a good {010} cleavage and conchoidal fracture. Varennesite is biaxi al positive, with indices of refraction alpha 1.532(2), beta 1.540(2), gamma 1.550(2), 2V(meas.) = 89(1)degrees and 2V(calc.) = 84.1 degrees . It is orthorhombic, space group Cmcm, with a 13.447(3), b 15.022(5), c 17.601(4) Angstrom and Z = 4. The strongest X-ray powder-diffractio n lines [d in Angstrom(I)(hkl)] are: 10.049(100)(110), 8.823(50)(002), 5.025(20)(113), 4.138(10)(132), 3.806(20)(223), 3.659(10)(204), 3.148 (10)(402), and 2.718(50)(423). Crystals are platy on {010}, with forms {100}, {010} and {101}. Electron-microprobe analyses gave Na2O 19.25, CaO 0.11, K2O 0.21, SrO 0.15, MnO 6.50, FeO 3.25, TiO2 2.56, MgO 0.03 , SiO2 48.26, Al2O3 0.48, SO3 1.02, Cl 2.20, and H2O 18.47 (calculated by stoichiometry from the crystal-structure analysis), O=Cl -0.50, to tal 102.00 wt.%, yielding the empirical formula (Na7.64Ca0.02K0.06Sr0. 02)(Sigma 7.74)(Mn1.13Fe0.56Ti0.39Mg0.01)(Sigma 2.09) (Si9.88Al0.12)(S igma 10.00)O-25.31[Cl-0.76(OH)(1.24)](Sigma 2.00). 12H(2)O(SO3)(0.16), ideally Na8Mn2Si10O25(OH,Cl)(2) . 12H(2)O; D-calc = 2.32 g/cm(3), D-m eas. = 2.31 g/cm(3). The structure has been refined to R = 7.9 and R(w ) = 6.3%. It consists of alternating, undulating layers of silicate te trahedra and Na and Mn octahedra. The structure has similarities to th e monophyllosilicate structures of manganpyrosmalite, bementite and ap ophyllite, but is unique in topology. Varennesite crystallized late in the formation of the Saint-Amable sill and appears to be the most Na- rich member of similar phases in peralkaline rocks.