Order–disorder–reorder process in thermally treated dolomite samples: a combined powder and single-crystal X-ray diffraction study

Abstract

A combined powder and single-crystal X-ray diffraction analysis of dolomite [CaMg(CO 3 ) 2 ] heated to 1,200°C at 3 GPa was made to study the order-disorderreorder process. The order/disorder transition is inferred to start below 1,100°C, and complete disorder is attained at approximately 1,200°C. Twinned crystals characterized by high internal order were found in samples annealed over 1,100°C, and their fraction was found to increase with temperature. Evidences of twinning domains combined with probable remaining disordered portions of the structure imply that reordering processes occur during the quench. Twin domains are hereby proposed as a witness to thermally induced intra-layer-type cation disordering.

Potentially Toxic Elements in Ultramafic Soils: A Study from Metamorphic Ophiolites of the Voltri Massif (Western Alps, Italy)

“…(PANalytical, Almelo, The Netherlands) software. For the quantitative analyses, Rietveld refinements were performed with General Structure Analysis software (EXPGUI-GSAS) [59][60][61][62] using the crystallographic information file (CIF) from the American Mineralogist Crystal Structure Database [63].…”

Calibration of the dolomite clumped isotope thermometer from 25 to 350 °C, and implications for a universal calibration for all (Ca, Mg, Fe)CO3 carbonates

“…Here we report a ∆ 47 calibration for dolomite samples that were precipitated at known temperatures from 25 to 350°C -that is across almost the entire range of ambient temperatures in which dolomite and other carbonate minerals can be found -and that differ from one another in " intrinsic characteristics" such as formation mechanisms, bulk stable isotopic (δ (Redfern et al 1989;Dove and Powell 1989;Hammouda et al 2011, Zucchini et al, 2012 Historically, Graf and Golsdsmith (1956) first used the term protodolomite to describe the poorly ordered transitional stage through which dolomites went during hydrothermal synthesis in the laboratory. Since then, protodolomite is usually defined as rhombohedral carbonate with slight Caexcess and dolomite basal reflections, but lacking major superstructure-reflections of cation-ordering,…”

An Endorheic Lake in a Changing Climate: Geochemical Investigations at Lake Trasimeno (Italy)

“…Aqueous speciation calculations show that: lake waters are characterised by variable values of SI calcite , ranging from 0.07 to 1.12 (0.75 on average), oversaturation with respect to ordered dolomite and undersaturation with respect to quartz, chalcedony and amorphous silica; river waters are characterised by similar saturation conditions with respect to calcite and dolomite but are close to equilibrium with quartz and/or chalcedony; groundwater samples are practically at equilibrium with calcite and slightly oversaturated with respect to dolomite and quartz. Oversaturation with respect to ordered dolomite [40,41] is related to kinetic factors preventing dolomite precipitation [42,43] and is commonly observed both on groundwater and surface water in central Italy [44]. All water samples are subsaturated, to different degrees, with respect to fluorite, gypsum and halite.…”