Chiappinoite-(Y), Y2Mn(Si3O7)4, a new layer silicate found in peralkaline syenitic ejecta from the Á gua de Pau volcano, Azores

Abstract

Chiappinoite-(Y) (IMA2014-040), ideally Y_2Mn(Si_3O_7)_4, is a new mineral found in peralkaline syenitic ejecta from the Agua de Pau volcano, San Miguel Island, Azores District, Portugal. It was discovered by collector Luigi Chiappino of Milan, Italy, after whom the mineral is named. It occurs as colourless, thin to thick prisms, up to 1 mm long, with blunt chisel-like terminations, and exhibits the forms {100}, {010}, {001}, {110} and {011}. Crystals are transparent with vitreous lustre. The Mohs hardness is about 6 and the mineral is brittle with uneven fracture and one perfect cleavage on {001}. The measured and calculated densities are 3.09(2) and 3.073 g/cm^3, respectively. Chiappinoite-(Y) is biaxial (–) and the refractive indices (white light) are α = 1.590(1), β = 1.5978(10), γ = 1.5982(10); 2V_(meas) = 24(1)° and 2V_(calc) = 25° . The mineral exhibits no dispersion and is nonpleochroic. The optical orientation is X = c, Y = b, Z = a. The empirical formula on 28 O apfu is (Y_(1.17)Ce_(0.18)Dy_(0.10)Na_(0.10)Nd_(0.09)Er_(0.08)La_(0.07)Gd_(0.07)Yb_(0.06) Sm_(0.03)Pr_(0.02))_(Σ1.97)(Mn^(2+) _(0.61)Ca_(0.25)Fe^(2+)_(0.09))_(Σ0.95)Si_(12.07)O_(28). The eight strongest reflections in the X-ray diffraction pattern [d_(obs). in A (I) (hkl)] are: 9.84 (90) (002), 4.129 (52) (024), 3.977 (48) (114), 3.544 (100) (211,042,202), 3.203 (48) (222), 2.999 (71) (044,204), 2.478 (67) (310,046,206) and 2.065 (57) (multiple). The mineral is orthorhombic, Ibam, with ɑ = 7.5549(3), b = 15.2342(5), c = 19.6418(14) A, V = 2260.6(2) A^3 and Z = 4. The structure of chiappinoite-(Y) contains 3-tetrahedra-thick silicate layers consisting of sinuous batisite-like chains, above and below which are four-membered silicate rings. Eight-coordinate Y and Mn sites are located between the silicate layers. The structure is essentially identical to that of the synthetic phases PrNaSi_6O_(14) and NdNaSi_6O_(14), except that the latter phases have an additional 12-coordinate Na site within the “cage” in the silicate layer.