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

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) ų, and the chemical composition Pb_1.00(Ca_1.86Fe²⁺ _0.07Mn_0.04Cr³⁺_0.02) _Σ=1.99(Zn_1.00Si_1.00O4)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.