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Published February 2015 | metadata_only
Journal Article

Chiappinoite-(Y), Y_2Mn(Si_3O_7)_4, a new layer silicate found in peralkaline syenitic ejecta from the Água de Pau volcano, Azores


Chiappinoite-(Y) (IMA2014-040), ideally Y_2Mn(Si_3O_7)_4, is a new mineral found in peralkaline syenitic ejecta from the Água de Pau volcano, Sãn 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 Å (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) Å, V = 2260.6(2) Å^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.

Additional Information

© 2014 E. Schweizerbart'sche Verlagsbuchhandlung. Received 18 September 2014. Modified version received 8 October 2014. Accepted 9 October 2014. Published online 13 November 2014. The paper benefited from comments by two anonymous reviewers. The Caltech EMP analyses were supported by a grant from the Northern California Mineralogical Association. The remainder of this study was funded by the John Jago Trelawney Endowment to the Mineral Sciences Department of the Natural History Museum of Los Angeles County.

Additional details

August 22, 2023
August 22, 2023