A mineral from Ettringer Bellerberg (Eifel, Germany) proved to be a polytype of the important ceramic-phase mullite termed mullite-2c, with – similar to sillimanite – doubling of the c lattice parameter due to strong (Si,Al) order in tetrahedral diclusters and – similar to mullite – presence of oxygen vacancies and tetrahedral triclusters due to Al / Si > 1 in diclusters. Crystals were characterised using single-crystal XRD, electron microprobe analysis (EMPA) and spindle-stage optical methods.

Type material for heliophyllite, preserved in the Swedish Museum of Natural History in Stockholm, was re-investigated through a combined EPMA (electron probe X-ray microanalysis), Raman, and X-ray powder diffraction (XRPD) and single-crystal study. EPMA chemical data, together with Raman and single-crystal structural studies, point to heliophyllite being identical to ecdemite. XRPD synchrotron data highlight the presence of a minor quantity of finely admixed finnemanite in the analyzed material.