Werdingite from a pegmatite at Almgjotheii, Rogaland, Norway: The role of iron in a borosilicate with a mullite-type structure

Abstract

Werdingite from a pegmatite on Almgjotheii in the aureole of the Rogaland anorthosite complex, Norway was reported in 1998 by Grew and others as the “Fe-dominant analogue” because of its high total Fe (to 8.00 wt% as FeO) and atomic Fe/(Mg + Fe) ratio ( X Fe to 0.75). New electron microprobe analyses (Be from 1998 ion microprobe data) of one crystal (sample HE138B2) give Fe 1.27 Mg 0.48 Al 14.44 Be 0.10 B 4.18 Si 3.61 O 37 . Optically, this crystal has α = 1.619(2), β = 1.652(2), γ = 1.658(2), 2 V x = 43(2)°. Its space group is P 1; a = 7.9696(2) A, b = 8.1559(3) A, c = 11.3607(3) A, α = 110.364(1)°, β = 110.881(1)°, γ = 84.670(2)°, V = 646.41(3) A 3 , D calc = 3.11 g/cm 3 , Z = 1. Comparison with literature data shows that the refractive indices of werdingite increase with increasing X Fe , whereas the a cell parameter decreases; the c cell parameter decreases with (B + Be) content. A crystal structure refinement ( R 1 = 0.057) gave 1.21 Fe occupying the one tetrahedral and two 5-coordinated sites present in type werdingite plus a new, largely vacant tetrahedral site, FeX. Fe is not dominant at any site. As we have no data on Fe valence, we cannot determine whether divalent cations are dominant at Mg5A, i.e. , we cannot show that HE138B werdingite is a Fe-dominant analogue of type werdingite. Werdingite in HE138B is intergrown with boralsilite, ideally Al 16 B 6 Si 2 O 37 , also related to mullite. B varies inversely with Si in a single trend for both minerals (per 37 O): 5.91–5.11 B and 1.95–2.56 Si in boralsilite [6] and 4.80–4.03 B and 3.09–3.74 Si in werdingite, whereas Al and (Mg + Fe) show no trend with Si in werdingite. Subparallel prisms of werdingite and boralsilite have a ladder-like appearance due to exsolution of one mineral in lamellae oriented perpendicular to prism length of the other, i.e. , perpendicular to the mullite-type chains of Al octahedra. Matching the two structures in this plane is possible: the pseudo-tetragonal cell perpendicular to the mullite-type chains is 7.65 × 7.42 A in werdingite vs. 7.54 × 7.38 A in boralsilite. Moreover, the two structures have in common Si 2 O 7 dimers and five-coordinated sites. Thus, our data are consistent with the suggestion of limited solid solution between the two minerals with subsequent exsolution of one from the other.