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Incorporation of Co in dolomite structure: coupled single crystal and EPMA investigations of cobaltoan dolomites from Tenke-Fungurume, D.R. of Congo
No full textCobaltoan dolomite from Tenke-Fungurume, D.R. of Congo, can host up to 20% mol Co (Barton et al., 2014).
Incorporation of Co in dolomite is definitely more relevant than in cobaltoan calcite, because of the closeness in ionic
radius between Mg2+ (0.72) and Co2+ (0.745) (Shannon, 1976). A set of crystals from this locality was studied through
coupled EPMA and single crystal structural refinements to investigate structural changes induced by Co incorporation
in dolomite structure. As expected, Co incorporation reflects in a clear direct trend in cell volume expansion, with the a
cell parameter more sensible than the c cell parameter to Co content.
Dolomite structure can be described (Reeder & Wenk, 1983) through “layers” of Me2+ alternating with “layers”
hosting CO3 groups along the c axis, approximating an hexagonal close packing. Both the Mg and Ca polyhedra are
trigonally distorted, namely elongated along the threefold axis. A scrutiny of Me-O bond distances shows a direct
correlation between the volume of (Mg,Co) coordination polyhedron and the unit cell volume, whereas the volume of
Ca polyhedron is inversely related to the Co content hosted in Mg polyhedron. In fact, when the (Mg,Co) polyhedron
expands, the oxygen atom is shifted towards Ca cation, resulting in a contraction of the Ca coordination polyhedron.
The distortion of Ca and (Mg,Co) coordination polyhedra is less pronounced in “basal” edges, namely those
polyhedron edges lying perpendicular to the c axis, respect to “lateral” edges. The expansion of the “basal” edges of the
Mg polyhedron with increasing Co content is more relevant than the slight expansion of the Mg lateral edges; the same
holds for the contraction of Ca polyhedron. This is in agreement with the pronounced variation of the a cell parameter,
directly related to the length of the “basal” edges. In dolomites, because of the difference in length between the longer
Ca-O and shorter (Mg,Co)-O bonds, a “rotation” of the octahedra and of the CO3 groups along the ternary axis, is
necessary to accomplish a fit. Their orientation in calcite structure, hosting Ca octahedra all of the same size, can be
assumed as starting zero point for comparison. Following Reeder & Wenk (1983) the mean rotation angle of CO3
groups is a convenient measure of the net rotations of all the components: in cobaltoan dolomites the net rotation angles
of CO3 groups decreases with increasing Co content
Crystal chemical investigations on rare zirconium-niobium silicates of the Alfred Lacroix collection of the Muséum National d’Histoire Naturelle (Paris)
No full textHeliophyllite: A discredited mineral species identical to ecdemite
Get PDFThe 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, explaining the presence of some additional diffraction peaks, not indexable with the ecdemite unit cell, reported in the literature. The discreditation of heliophyllite has been approved by the IMA Commission on New Minerals and Mineral Names (proposal 19-H, 2019)
Zoisite-(Pb), a new orthorhombic epidote-related mineral from the jakobsberg mine, värmland, sweden, and its relationships with hancockite
No full textThe new mineral zoisite-(Pb), ideally CaPbAl3 (SiO4)(Si2 O7)O(OH), was discovered in a sample from the Jakobsberg manganese-iron oxide deposit, Värmland, Sweden. Zoisite-(Pb) is found as pale pink subhedral prisms elongated on [010], up to 0.3 mm in size, associated with calcite, celsian, diopside, grossular, hancockite, hyalophane, native lead, phlogopite, and vesuvianite. Associated feldspars show one of the highest PbO contents (~7–8 wt%) found in nature. Electron-microprobe analysis of zoisite-(Pb) point to the empirical formula (Ca1.09 Pb0.86 Mn2+ 0.01Na0.01)∑1.97 (Al2.88 Fe3+ 0.10 Mn3+ 0.04)∑3.02 Si3.00 O12 (OH)1.00 . The eight strongest diffraction lines [dobs, Iobs, (hkl)] are 8.63 s (101), 8.11 mw (200), 4.895 m (011), 4.210 m (211), 3.660 s (112, 311), 3.097 mw (312), 2.900 s (013), and 2.725 m (511). Zoisite-(Pb) is isostructural with zoisite and its crystal structure was refined up to R1 = 0.0213 for 2013 reflections with Fo > 4σ(Fo). Pb shows a stereochemically active lone pair leading to a lopsided distribution of its coordinating oxygens. A full chemical and Raman characterization of zoisite-(Pb) and of the Pb-bearing epidote hancockite is reported, together with an improved crystal structural model of hancockite, refined up to R1 = 0.0254 for 2041 reflections with Fo > 4σ(Fo). The effects of the incorporation of Pb in the crystal structure of zoisite-(Pb), hancockite, and related synthetic and natural phases are described and discussed
Mn-rich graftonite, ferrisicklerite, staněkite and Mn-rich vivianite in granitic pegmatite at Soà ̈ Valley, Central Alps, Italy
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Crystal chemical study of ecdemite from Harstigen, a new natural member of the layered lead oxyhalides structural group
Get PDFEcdemite from Harstigen, Värmland, Central Sweden was studied through Raman and
FTIR spectroscopy, electron microprobe techniques, synchrotron powder and single
crystal diffraction. The ideal mineral formula proposed by Palache et al. (1951),
Pb6Cl4As2O7, is confirmed. In contrast to previous suggestions, the present study
demonstrates that the mineral is monoclinic, with space group P21 and unit-cell
parameters a = 10.8276 (38) Å, b = 10.8265 (21) Å, c = 6.9705 (14) Å, Β = 113.261°
(24). The crystal structural study of ecdemite shows that the mineral can be considered
as made up by regularly alternating, with 1:1 ratio, Cl layers and litharge-like defective
layers, also hosting As cations. These features indicate that ecdemite can be
considered as a new member of the of layered lead oxyhalides group
Credenza religiosa e virtù epistemologiche. Il contributo di Pouivet alla filosofia analitica della religione in Francia
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