Search

The name sigismundite has been reinstated from arrojadite-(BaFe) following the IMA Commission on New Minerals, Nomenclature and Classification (CNMNC) recommendations defined by Hatert et al. (2013) for the preservation of historical names. The name change was approved by the CNMNC, voting proposal 22-C. The new mineral symbol for sigismundite is Sigi. Sigismundite honours Pietro Sigismund (1874–1962), and this paper outlines his significant contributions to Italian mineralogy.

A reinvestigation of the type angastonite specimen from the Penrice marble quarry, South Australia, shows that the published powder X-ray diffraction pattern for the mineral corresponds to a mixture of crystalline phases plus an amorphous phase. The published formula for the mineral, CaMgAl2(PO4)2(OH)4⋅7H2O, has been found to correspond to the amorphous phase. A proposal for the redefinition of angastonite as an amorphous phase has been approved by the IMA CNMNC. Amorphous angastonite is isotropic with an index of refraction in the range 1.33 to 1.40 and a measured density of 1.57(2) g cm−3. It is formed from incongruent leaching of minyulite in circulating Mg- and Ca-bearing solutions. The amorphous phase undergoes local recrystallisation to form penriceite and two other new crandallite-derivative layer phases. A cluster model for the local structure in the amorphous mineral is proposed based on crandallite-type segments. A brief review of amorphous minerals is given.

Synthetic and naturally occurring forms of tricopper orthotellurate, (Cu3TeO6)-Te-II-O-IV (the mineral mcalpineite) have been investigated by 3D electron diffraction (3D ED), X-ray powder diffraction (XRPD), Raman and infrared (IR) spectroscopic measurements. As a result of the diffraction analyses, (Cu3TeO6)-Te-II-O-IV is shown to occur in two polytypes. The higher-symmetric (Cu3TeO6)-Te-II-O-IV-1C polytype is cubic, space group 1a (3) over bar, with a = 9.537 (1) angstrom and V = 867.4 (3) angstrom(3) as reported in previous studies. The 1C polytype is a well characterized structure consisting of alternating layers of (CuO6)-O-II octahedra and both (CuO6)-O-II and (TeO6)-O-VI octahedra in a patchwork arrangement. The structure of the lower-symmetric orthorhombic (Cu3TeO6)-Te-II-O-IV-2O polytype was determined for the first time in this study by 3D ED and verified by Rietveld refinement. The 2O polytype crystallizes in space group Pcca, with a = 9.745 (3) angstrom, b = 9.749 (2) angstrom, c = 9.771 (2) angstrom and V = 928.3 (4) angstrom(3) . High-precision XRPD data were also collected on (Cu3TeO6)-Te-II-O-IV-2O to verify the lower-symmetric structure by performing a Rietveld refinement. The resultant structure is identical to that determined by 3D ED, with unit-cell parameters a = 9.56157 (19) angstrom, b = 9.55853 (11) angstrom, c = 9.62891 (15) angstrom and V = 880.03 (2) angstrom(3) . The lower symmetry of the 2O polytype is a consequence of a different cation ordering arrangement, which involves the movement of every second (CuO6)-O-II and (TeO6)-O-VI octahedral layer by (1/4, 1/4, 0), leading to an offset of (TeO6)-O-VI and (CuO6)-O-II octahedra in every second layer giving an ABAB* stacking arrangement. Syntheses of (Cu3TeO6)-Te-II-O-IV showed that low-temperature (473 K) hydrothermal conditions generally produce the 2O polytype. XRPD measurements in combination with Raman spectroscopic analysis showed that most natural mcalpineite is the orthorhombic 2O polytype. Both XRPD and Raman spectroscopy measurements may be used to differentiate between the two polytypes of (Cu3TeO6)-Te-II-O-IV. In Raman spectroscopy, (Cu3TeO6)-Te-II-O-IV-1C has a single strong band around 730 cm(-1), whereas (Cu3TeO6)-Te-II-O-IV-2O shows a broad double maximum with bands centred around 692 and 742 cm(-1).