The crystal structure of esperite, with a revised chemical formula, PbCa2(ZnSiO4)3, isostructural with beryllonite

Kimberly T. Tait, Hexiong Yang, Robert T. Downs, Chen Li, William W. Pinch

Research output: Contribution to journalArticle

5 Scopus citations

Abstract

Esperite from Franklin, New Jersey, was first described by Moore and Ribbe (1965) as monoclinic with a well-developed "superlattice" a = 2 × 8.814(2) Å, b = 8.270(3) Å, c = 2 × 15.26(1) Å, β ≈ 90°, space group P21/n (subcell), and the chemical formula PbCa3(ZnSiO4)4. They attributed "superlattice" reflections to the ordered distributions of Pb and Ca cations over four beryllonite-type subcells for esperite with the Ca:Pb ratio greater than 2:1. We examined two esperite fragments from the type sample using single-crystal X-ray diffraction, electron microprobe analysis, and Raman spectroscopy. Although both fragments have Ca:Pb ≈ 1.8, one exhibits the "superlattice" reflections as observed by Moore and Ribbe (1965), whereas the other does not. The sample without "superlattice" reflections has unit-cell parameters a = 8.7889(2), b = 8.2685(2), c = 15.254(3) Å, β = 90.050(1)°, V = 1108.49(4) Å3, and the chemical composition Pb1.00(Ca1.86Fe2+ 0.07Mn0.04Cr3+0.02) Σ=1.99(Zn1.00Si1.00O4)3. Its crystal structure was solved in space group P21/n (R1 = 0.022). Esperite is isostructural with beryllonite, NaBePO4, and its ideal chemical formula should, therefore, be revised to PbCa 2(ZnSiO4)3, Z = 4. The ZnO4 and SiO4 tetrahedra in esperite share corners to form an ordered framework, with Pb2+ occupying the nine-coordinated site in the large channels and Ca2+ occupying the two distinct octahedral sites in the small channels. The so-called "superlattice" reflections are attributed to triple twins, a trilling of ∼60° rotational twinning around the b axis, similar to those observed in many other beryllonite-type materials. A phase transformation from a high-temperature polymorph to the esperite structure is proposed to be responsible for the twinning formation.

Original languageEnglish (US)
Pages (from-to)699-705
Number of pages7
JournalAmerican Mineralogist
Volume95
Issue number5-6
DOIs
StatePublished - May 1 2010

Keywords

  • Beryllonite
  • Crystal chemistry
  • Esperite
  • Pb-Zn silicates
  • X-ray diffraction

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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