Perettiite-(Y), Y³⁺₂Mn²⁺₄Fe²⁺[Si₂B₈O₂₄], a new mineral from Momeik, Myanmar

Author(s)

Rosa Micaela Danisi, Thomas Armbruster, Eugen Libowitzky, Hao A.O. Wang, Detlef Günther, Mariko Nagashima, Eric Reusser, Willy Bieri

Abstract

The new mineral perettiite-(Y), end-member formula Y

³⁺

₂Mn

²⁺

₄Fe

²⁺[Si

₂B

₈O

₂₄], from pegmatites in the region of Momeik, north of Mogok, Myanmar, occurs as yellow needles enclosed in phenakite crystals. The mineral was named after the mineralogist and gemologist Dr. Adolf Peretti. The empirical formula using LA-ICP-MS data (based on 24 O pfu) is Y

_2.06Ln

_0.53Zr

_0.02Th

_0.01Mn

_3.24Ca

_0.38Fe

_0.71Mg

_0.07Al

_0.11Li

_0.22Si

_1.95B

_7.44Be

_0.31O

₂₄. Calculation of the H2O content using IR absorption data yields a maximumhydroxyl/water content equivalent to 0.1 wt.% H

₂O. The strongest lines in the powder pattern [d in Å (I

_meas) (hkl)] are 4.63 (52) (010), 4.08 (28) (301,103), 3.74 (20) (210), 3.05 (100) (113, 311, 303), 2.64 (67) (410,014), 2.54 (60) (313), 2.12 (23) (600,006), 1.87 (33) (420,024), 1.84 (52) (415, 323), 1.57 (20) (026,620), 1.44 (25) (133, 331). The perettiite-(Y) crystals show a tetragonal X-ray diffraction pattern but the structure could only be solved as a 50/50 pseudo-merohedral orthorhombic twin with the a and c axes having the same length. The structure, refined to R1 = 0.017 (space group Pmna with a = 12. 8252(5), b = 4. 6187(2), c = 12.8252(5) A, V = 759.71(5) A, Z = 2), is characterized by two eight-coordinated sites: one dominated by Y and lanthanoids (Ln) and the other by Mn

²⁺ (with additional Ca

²⁺ and Y

³⁺). An octahedral site is occupied by (Fe

²⁺, Mg) with additional Li

⁺. These cation sites form an interlayer between two tetrahedral sheets of Si

₂B

₈O

₂₄ composition. Boron shows minor replacement by beryllium. Arrangement of 4-, 5- and 8-membered rings within (010) form the borosilicate tetrahedral sheets. Perettiite-(Y) is brittle, with irregular fracture and (010) cleavage; the estimated Mohs hardness is ∼ 7. Using the empirical formula, the density was estimated to be 4.533 g cm

^-3. Perettiite-(Y) is biaxial, α = 1. 82(1), γ = 1.84(1) (589 nm).

Organisation(s)

Department of Mineralogy and Crystallography

External organisation(s)

Eidgenössische Technische Hochschule Zürich, Yamaguchi University, GRS Gemresearch Swisslab AG, Universität Bern

Journal

European Journal of Mineralogy: an international journal of mineralogy, geochemistry and related sciences

Volume

27

Pages

793-803

No. of pages

11

ISSN

0935-1221

DOI

https://doi.org/10.1127/ejm/2015/0027-2483

Publication date

08-2015

Peer reviewed

Yes

Austrian Fields of Science 2012

105116 Mineralogy, 105113 Crystallography

Keywords

ASJC Scopus subject areas

Geochemistry and Petrology

Portal url

https://ucrisportal.univie.ac.at/en/publications/99859aa5-2888-4e84-9949-d410d2029614