Magnesiofoitite, square (Mg2Al) Al-6 (Si6O18) (BO3)(3) (OH)(4), a new alkali-deficient tourmaline

Magnesiofoitite is a new tourmaline species from an alteration zone in a si licified porphyry developed in completely altered andesitic to dacitic volc anic rocks at Kyonosawa, Mitomi-mura, Higashi-Yamanashi-gun, Yamanashi-ken Prefecture, Honshu, Japan. It occurs as bluish grey individual crystals and felted masses on fracture and void surfaces; individual crystals average s imilar to 5 mu m wide and 50 mu m long, and a few crystals approach 15 mu m wide and 1 mm long. It is brittle, H = 7, D-calc = 2.995 g/cm(3). In plane -polarized light, magnesiofoitite is pleochroic, O = grey-blue, E = pale la vender, with moderate absorption, omega > epsilon; it is uniaxial negative, omega = 1.624, epsilon = 1.650, trigonal, space group R3m, in the hexagona l setting, with a 15.884(4), c 7.178(3) Angstrom, V= 1568.0(6) A(3), Z = 3. The strongest six X-ray-diffraction lines [d in Angstrom(I)(hkl)] are: 3.9 69(100)(220), 2.567(100)(051), 4.211(90)(211), 2.949(70)(122), 6.366(60)(10 1), and 3.470(60)(012). A chemical analysis gave (in wt.%), SiO2 38.27, Al2 O3 40.17, FeO 0.97, MgO 6.15, Na2O 0.70, B2O3(calc.) 11.09, H2O (calc.) 3.8 2, sum = 101.17 wt.%; P: Ti, Ca, Mn, Zn, K and F were not detected. The for mula unit, calculated for 6 Si apfu (atoms per formula unit) with B = 3.0 a pfu and OH = 4.0 apfu, is X(rectangle(0.79)Na(0.21)) (Y)(Mg1.44Fe0.13Al1.42 ) Al-Z(6) (Si6O18) (BO3)(3) (OH)(4), With the ideal end-member formula rect angle (Mg2Al) Al-6 (Si6O18) (BO3)(3) (OH)(4), where O represents a vacancy. Thus magnesiofoitite is an X-site-vacant tourmaline; it is derived from fo itite, rectangle (Fe22+Al) Al-6 (Si6O18) (BO3)(3) (OH)(4) by the homovalent substitution Mg-Y --> Fe-Y(2+), and from dravite, Na Mg-3 Al-6 (Si6O18) (B O3)(3) (OH)(4), by the heterovalent substitution (X)rectangle + Al-Y --> Na -X + Mg-Y. The new species and name were approved by the CNMMN of the IMA.