Multiphase carbonatite-related magmatic and metasomatic processes in the genesis of the ore-hosting dolomite in the giant Bayan Obo REE-Nb-Fe deposit
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Carbonatite
Metasomatism
Recrystallization (geology)
Abstract The Raman spectra of calcite, Mg‐calcite, and dolomite were measured under ambient and high pressure–temperature ( P ‐ T ) conditions using a hydrothermal diamond anvil cell, for the purpose of developing new pressure sensors suited for experiments investigating the physicochemical properties of carbonate minerals. By fitting the Raman vibrational frequencies as functions of pressure and temperature, pressure ( P , in MPa) can be determined from relative frequency shifts (in cm −1 ) of the symmetric stretching ( ν 1 ) and librational ( ν L ) lattice vibrations of calcite: P = 229(1) × ( ν 1 calcite, HP − ν 1 calcite, ref ), P = 162(1) × ( ν L calcite, HP − ν L calcite, ref ), or from the translational ( ν T ) and librational ( ν L ) lattice vibrations of dolomite: P = 371(3) × [( ν L dolomite, HP − ν T dolomite, HP ) − ( ν L dolomite, ref − ν T dolomite, ref )], where ν ref is the value under ambient P ‐ T conditions. Under elevated temperatures, correction for the effect of temperature ( T , in °C) on the Raman frequency shifts can be accomplished through ν 1 calcite, HT = ν 1 calcite, ref − 6.8(5) × 10 −6 × T 2 –0.0051(3) × T + 0.12, ν L calcite, HT = ν L calcite, ref − 1.07(12) × 10 −5 × T 2 –0.00341(7) × T + 0.76, ν L dolomite, HT − ν T dolomite, HT = ( ν L dolomite, ref − ν T dolomite, ref ) − 0.0198(3) × T + 0.45. Application of the calcite and dolomite Raman pressure sensors should be made on condition that no phase transitions or breakdown reactions occur under high P ‐ T conditions, with errors of ±50 MPa and ± 5% at pressures below and above 1.0 GPa, respectively. Results from an additional experiment showed that pressures determined using alternate Raman pressure calibration methods were self‐consistent and in excellent agreement with the results determined using the ν 1 Raman peak shifts of aragonite.
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Trace element geochemistry for dolomitic limestones was carried out by chemical separation of the constituent calcite and dolomite. A five-minute treatment for fine-grained dolomitic limestone with 0.5 M acetic acid differentially dissolves the calcite, but 82–90% of dolomite remained undissolved. The fraction of dissolved dolomite contributed 2–17% to the calcite fraction, depending on the calcite/dolomite ratios of the samples. The residue of the acetic acid treatment was treated with 0.5 M HCl and an almost pure dolomite fraction was obtained. Comparison of the trace element concentrations in the calcite and dolomite fractions for four dolomitic limestones revealed that the REE abundances in both the fractions were similar, while the Fe, Mn, P, and Al concentrations in the dolomite fractions were higher than those in the calcite fractions. The method developed in this study can be used for trace element studies of dolomitic limestones.
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Fraction (chemistry)
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Experimental results are presented which support conclusions derived from a theoretical analysis of the log V vs. t plot, representing the rate of solution of dolomite consisting of a number of crystals. Experimentally determined rates of solution of dolomite and calcite are presented. The intercept method for determining the proportion of dolomite and calcite in mixtures of the two is evaluated on the basis of the theoretical and experimental results.
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The determination is based on the difference in rates of solution of calcite and dolomite. The CO 2 evolved from reaction of a mixture of calcite and dolomite in excess HCl is measured at frequent intervals until the reaction is completed. The logarithm of the amount of CO 2 equivalent to unreacted carbonate is plotted against time. Following the rapid and complete solution of calcite, the curve for dolomite assumes a much smaller slope and is linear for a period of time. Extrapolation of this linear portion of the curve to zero time gives the amount of CO 2 equivalent to the dolomite initially present. The CO 2 from calcite is obtained by difference from the total CO 2 . The amounts of calcite and dolomite found in a number of prepared mixtures by this intercept method were in good agreement with the amounts known to be present. Several samples of limestone and calcareous soil were analysed by the proposed method and the results were compared with X-ray diffraction and chemical data.
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Carbon fibers
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X-ray diffraction has been widely used in analyzing Ca-Mg carbonates. Compositions of biogenic and inorganic (Ca,Mg)CO3 crystals are often calculated by comparing their d104 values with published empirical curves. However, previous studies suggested that these curves do not apply to very high-Mg calcite and disordered dolomite. Based on synthesized high-Mg calcite and disordered dolomite, a new empirical curve between values of magnesian calcite d104 and MgCO3 content in the calcite-disordered dolomite solid-solution series is constructed. This new curve is consistent with the significant cell parameter changes accompanying the Mg-Ca cation disorder in dolomite, and it can help the characterization of the MgCO3 content of both natural and synthetic magnesian calcite and disordered dolomite, especially for the mineral mixtures that are not suitable for other analysis methods.
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