Crystal structure and composition of hiärneite, Ca2Zr4Mn3+SbTiO16, and constitution of the calzirtite group

Dan Holtstam,Luca Bindi

doi:10.1180/mgm.2022.19

Abstract

AbstractThe crystal structure of hiärneite has been refined from single-crystal X-ray diffraction data (λ = 0.71073 Å) on type material from Långban, Värmland, Sweden. The refinement converged to R1 = 0.046 based on 1073 reflections with F2 > 4σ(F2). The tetragonal unit cell, space group I41/acd, has the parameters a = 15.2344(6) Å and c = 10.0891(6) Å with Z = 8. The mineral is isostructural with calzirtite, ideally Ca2Zr5Ti2O16, with a structural topology derived from fluorite. In hiärneite, Mn3+ is ordered at a 4- to 8-fold coordinated site (with a distorted polyhedral coordination figure), without the atom splitting encountered at the corresponding Zr-dominated site of calzirtite. The end-member formula for hiärneite is established as Ca2Zr4Mn3+SbTiO16. The calzirtite group, with calzirtite, hiärneite and tazheranite (cubic ZrO2-x), has been approved by the IMA–CNMNC.

Highlights

Hiärneite, (Ca,Na,Mn2+)2(Zr,Mn3+)5(Sb,Ti,Fe3+)2O16, was described as a new mineral (Holtstam, 1997) from the famous Fe–Mn–(Ba–Pb–As–Sb–Be–B) deposit at Långban, in Värmland, Sweden (Holtstam and Langhof, 1999)
Hiärneite was the first mineral known to have an essential combination of Zr and Sb in its formula
Crystal data Ideal formula Crystal dimensions Crystal system, space group Temperature (K) a, c (Å) V (Å3) Z Data collection Crystal description Instrument Radiation type, wavelength (Å) θ range (°) Absorption correction Number of measured, independent and observed reflections Rint Indices range of h, k, l Refinement Refinement

Summary

Introduction

Hiärneite, (Ca,Na,Mn2+)2(Zr,Mn3+)5(Sb,Ti,Fe3+)2O16, was described as a new mineral (Holtstam, 1997) from the famous Fe–Mn–(Ba–Pb–As–Sb–Be–B) deposit at Långban, in Värmland, Sweden (Holtstam and Langhof, 1999). It occurs in a phlogopite-rich skarn with magnetoplumbite, fluorapatite, oxyplumboroméite, pyrophanite and jacobsite (Fig. 1), and was interpreted as a product of metasomatism and regional metamorphism. Given the close similarity in terms of unit-cell dimensions and crystal symmetry, the crystal structure was refined starting from the atomic coordinates of calzirtite, ideally Ca2Zr5Ti2O16 (Rossel 1982; Sinclair et al, 1986; Rastsvetaeva et al, 1995; Jafar et al, 2016). The mean electron numbers obtained for each site were modelled considering the observed bond distances and the electron microprobe analysis (see below) These proportions were fixed in subsequent refinement cycles (see Table 2 – constrained refinement).

Site Site occupancy x y z

Crystal structure

Chemical composition

Dan Holtstam and Luca Bindi

ΣM Ideal