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Crystal structure of lizardite 1T from Elba Island, Italy
No full textEuhedral lizardite-1T occurs in the Monte Fico quarries, Elba Island, Italy. Unit-cell parameters, for two crystals, each with space group P31m, are a = 5.338(4) and c = 7.257(6) and a = 5.330(4) and c = 7.269(6) Å. The crystal structures were refined to Rtot = 0.034 and 0.046, using 251 and 849 independent reflections, respectively. The topology determined from previous refinements is confirmed. -from Author
Marianoite, a new member of the cuspidine group from the Prairie Lake silicocarbonatite, Ontario: discussion
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Enstatite chemical composition and microstructures in the La Villa H4 chondrite
No full textLa Villa is an unshocked H4 chondriate. Chemical compositions require crystallization at temperatures >1250 °C for enstatite and >1211 °C for augite. Widespread (100) polysynthetic twins and (001) contraction cracks in enstatite indicate crystallization as protoenstatite, inverted to either ortho- or clinoenstatite or both on cooling. High-resolution transmission electron microscopy shows a range of orthoclinoenstatite intergrowths: heavily faulted clinoenstatite in radial and poikilitic chondrules, almost regular orthoenstatite in a microgranular chondrule and in the matrix. In the former, the clinoenstatite lamellae are both even or odd multiples of the 9 A periodicity, a few unit cells thick, twinned and interleaved with minor orthoenstatite. In the latter, orthoenstatite lamellae are regularly stacked for more than 2000 A. Localized annealing effects, reversing clinoenstatite to orthoenstatite, are revealed by 'U-shaped' and 'Z-shaped' terminations. The variable microstructure suggest different cooling rates for the different chondrule types, soon after the liquidus-to-solidus transition (1200 to 1300 °C) but prior to accretion. In particular, clinoenstatite-rich crystals from radial and poikilitic chondrules give cooling rates on the order of 100 and 10 °C/h. Comparisons with previous works on dynamic crystallization experiments and orthopyroxene Fe-Mg cation ordering indicate a nonlinear cooling path from the high chondrule formation temperatures to a postaccretionary low-temperature (340-480 °C) evolution
Crystal chemistry of meteoritic kirschsteinite
No full textThe crystal structure of two different crystals (Fa(39)Fo(11)La(50) and Fa(35)Fo(14)La(51), respectively) are refined (orthorhombic unit cell parameters: a = 4.877 and 4.875, b = 11.166 and 11.164, c = 6.448 and 6.447 Angstrom), yielding similar results. In particular, the geometry of the M2 and T coordination polyhedra does not change with respect to other calcium bearing olivines, whereas M1 is sensitive to the site population
Mesh textures and bastites in the Elba retrograde serpentinites
No full textRetrograde serpentinized peridotites, belonging to the Internal Ligurid terrains, occur in eastern Elba island, Italy The mesh texture (with pyroxene bastites) is the typical textural arrangement. The mesh rims are apparently fibrous, with negative optical elongation; cores range from isotropic to microgranular. Optical and chemical determinations indicate harzburgite (75 % olivine and 25 % pyroxene) as the protolith for Elba serpentinites, with characteristics similar to mantle peridotites from ocean basins.
Microstructural analysis, led by transmission electron microscopy, indicates that the mesh rims, rather than being actually fibrous, consist of elongated lizardite crystals, having sharp triangular sections with apices pointing toward the mesh center; poorly crystallized material occurs in between adjacent lizardite sectors. The mesh cores consist of lizardite, chrysotile and polygonal serpentine, intermixed in variable amount and random mutual orientation. Bastites are generally poorly crystalline, formed by tiny lizardite lamellae with minor chrysotile.
Mesh rims and cores have chemical compositions variable from specimen to specimen, but almost constant within each specimen; cores are always aluminium-enriched with respect to the rims; when present, nickel (after olivine) occurs in the meshes. Bastites are chemically simpler than meshes, and, independently from their optical appearance, always have an aluminium-rich composition; chromium (after pyroxene) always characterizes bastites. Chemical data indicate absence of long range chemical fluxes during serpentinization.
The formation of the mesh texture (with bastites) is explained as a two-stage process. In stage I, the peridotite undergoes thermal fracturing and lizardite flakes start to decorate the pseudocubic fracture pattern (thus producing the mesh rim). Massive water arrival into the weakened peridotite induces stage III where serpentinization is completed forming the mesh cores and bastites. In stage I the reaction is thermodynamically controlled, in stage II the reaction is kinetically controlled
Exsolution and hydration of pyroxenes from partially-serpentinized harzburgites
No full textOrtho- and clinopyroxenes within partially-hydrated harzburgites from Elba and Val di Cecina (Italy) show lamellar exsolution textures and variable replacement by biopyriboles, talc-chlorite-serpentine mixed layers and serpentine. Chemical and geothermometric data suggest that the pyroxenes crystallized at 1240-1051°C, followed by subsolidus exsolution at slightly lower T (1145-1025°C for clinopyroxene lamellae + orthopyroxene matrix pairs and a 1033-982°C range for orthopyroxene lamellae + clinopyroxene matrix pairs). Investigation by transmission e lectron microscopy of exsolved enstatite and augite reveals a multistage hydration process. The first stage (highest T, probably in the amphibole stability field) leads to the formation of biopyribole lamellae within exsolved augite, leaving the enstatite matrix unaffected. The second stage (∼500-300°C) corresponds to the topotactic replacement of enstatite by layer silicates (talc + chlorite + serpentine, with (001)layer silicates parallel to (100)enstatite). Enstatite is also replaced by randomly oriented, poorly crystalline serpentine. The last hydration stage (<300°C) corresponds to the disappearence of augite and recrystallization of serpentine, leading to completely hydrated bastites with random lizardite lamellae, polygonal serpentine and minor chrysotile. © 2005 The Mineralogical Society
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