Low-temperature_nucleation_of_magnesite_and_dolomite.pdf
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1、 * Studies on irreversible geochemical reactions No.7 Low-temperature nucleation of magnesite and dolomite * By J. C. Deelman, Eindhoven With 4 figures in the text DEELMAN, J. C.: Low-temperature nucleation of magnesite and dolomite - N. Jb. Miner. Mh., 1999 (7): 289-302 , Stuttgart 1999. Abstract:
2、By way of duplicating an experiment described by Liebermann(1967) nucleation of magnesite, huntite and/or dolomite has been attained at temperatures between 313 K and 333 K and under atmospheric pressure. Essential to these experiments are fluctuations in pH value. After interrupting an experiment a
3、fter 1, 3, 5, or 8 of such fluctuations, the change from one or more metastable phases into the stable phase (magnesite or dolomite) could be followed. A theoretical explanation for these low-temperature syntheses can be found in stability relations. Ostwalds Rule stipulates the nucleation of a meta
4、stable phase before that of the stable phase. However fluctuations of sufficient amplitude and duration are capable of crossing the border between the metastable and the stable fields. As a result the stable phase will nucleate together with the metastable phase. Conditions opposing the subsequent g
5、rowth of the metastable phase (such as the slightly acidic conditions resulting from periodically introducing CO2 into the solution) will favour the continued growth of the stable phase. K e y w o r d s : Magnesite, dolomite, huntite, low-temperature syntheses. Magnesite The first time I observed th
6、e formation of magnesite was upon duplicating of Exp. No. 57 of Liebermann(1967). In my experiment M-211 0.918 Mol NaCl, 0.0316 Mol MgCl2.6 H2O , 0.018 Mol MgSO4.7 H2O and 0.020 Mol KCl (all chemicals are reagent grade) were dissolved in 330 cm3 distilled water and 2 mMol CaCO3 added later, was diss
7、olved by way of bubbling CO2 (industrial grade) through the suspension during 12 hours. Titration to pH = 8.00 took place with dilute ammonia. The next phase, the gradual escape of CO2 from the solution, involved heating the 500 cm3 Erlenmeyer flask on an electric plate to (in this case) 333 K durin
8、g 60 hours. As described by Liebermann(1967), the total experiment consists of repeating 14 times the dissolution phase, the titration and the phase of heating and gradual escape of CO2 . The pH of the solution was measured every time after bubbling CO2 through it (pH = 5,30 to 6,10 in this particul
9、ar test), upon titration, and at the end of each 60 hours phase of carbon dioxide escape Low-temperature nucleation of magnesite and dolomite J.C. Deelman 2 (pH = 8,34 to 8,85). The experiment lasts 42 days. The precipitate adheres the glass; the Erlenmeyer was washed several times with distilled wa
10、ter and after desiccation (at 303 K) the precipitate was scraped off. In X-ray diffraction (performed on a Philips X-ray diffractometer with Ni filtered Cu-K radiation; voltage 40 kV; current 20 mA; scanning speed 1o per minute.) magnesite together with a small amount of magnesium hydroxide carbonat
11、e was found. In order to remove the latter, the precipitate was partly leached in water, through which CO2 was bubbled during 12 hours. (Before leaching the precipitate weighed 1.5 gr; after leaching 0.4 gr.) The remaining precipitate was found to consist of pure magnesite (Fig. 1). Based on the 6 s
12、trongest lines (Table I) the cell parameters for the sample from experiment M-211 were calculated as ao = 46.41 nm and co = 150.94 nm. These unit cell dimensions may be compared with ao = 46.33 nm and co = 150.15 nm given in JCPDS-ICDD file no. 8-479, as well as with the ao = 46.41 nm and c o = 150.
13、93 nm measured for a sample of modern sedimentary magnesite from a salt lake in Turkey (Irion, 1970). When repeating the same experiment with a temperature of 313 K during the 60-hours “escape of CO2 phase“ (my experiment D-211) magnesite plus calcite and dolomite formed. Magnesite was found again a
14、fter duplicating the same Exp. No. 57 of Liebermann(1967) at 313 K (my experiment M-223), but once more together with a small amount of magnesium hydroxide carbonate (Fig. 2). In Liebermanns(1967) paper the suggestion is made, that either ammonia or a solution of sodium carbonate can be used for the
15、 titration. But when using a sodium carbonate solution, no magnesite forms. Nesquehonite plus Mg-calcite nucleate, when using a solution of sodium carbonate in Exp. No. 57 of Liebermann(1967) at 298 K. When duplicating Liebermanns Exp. No. 57 at 313 K with a concentrated solution of sodium carbonate
16、, magnesium hydroxide carbonate precipitates. Dolomite As part of a systematic investigation into the mechanism of dolomite formation all of the known claims on low-temperature synthesis have been duplicated by me. But the experiments yielded precipitates consisting at best of magnesium calcite. Ult
17、imately pure dolomite precipitated in an experiment, which was in fact a variation on the test described by Liebermann(1967). The addition of urea to Liebermanns Exp. No. 57 was found to favour the low-temperature nucleation of dolomite. In my experiment D-222, itself another duplication of Lieberma
18、nns Exp. No. 57 but with the addition of 16.65 mMol urea, dolomite sensu stricto together with some pure calcite formed (Fig. 3). X-Ray analyses were performed on a Philips X- ray diffractometer with nickel filtered copper radiation (voltage 40 kV; current 20 mA; but at slowed down scanning speed of
19、 1o per 3.34 min.). The presence of one of the superstructure reflections of dolomite (cf., Goldsmith but the only undetermined peak of the diffractogram from sample D-222 was found at 43.07o (or 2.09 nm) and may well belong to calcite. The use of urea has been inspired on the observations by Mansfi
20、eld(1980) on the occurrence of pure dolomite as bladder stones in a Dalmatian dog. The catalysis by urea is most probably related to the desorption of chlorine ions and the adsorption of carbonate ions. Chlorine ions (of Low-temperature nucleation of magnesite and dolomite - J.C. Deelman 3 the MgCl2
21、 used) are known to adsorb onto calcium carbonate surfaces even stronger than hydroxyl or bicarbonate groups (Douglas titration + 60 hours of CO2 escape), precipitates were obtained, that should represent subsequent stages in an overall reaction. In order to minimize possible variations in the compo
22、sition of the artificial brine, 6.6 dm3 of the solution used by Liebermann(1967) in his experiment No. 57 were prepared in advance. Each new experiment started out with adding 2 mMol CaCO3 (p.A., MERCK art. 2066) to 330 cm3 of the stock solution of artificial brine. Usually 12 hours of bubbling CO2
23、through the brine would suffice to completely dissolve all of the CaCO3 . Step-wise interruption was used in duplications of Liebermanns Exp. No. 57 at 333 (= exp. M-227) and 313 K (exp. D-228). The precipitates from each experiment were X-rayed in Low-temperature nucleation of magnesite and dolomit
24、e - J.C. Deelman 4 one uninterrupted session with identical instrument setting and calibration of the diffractometer. In exp. M-227 the initial precipitate consisted mainly of aragonite, little calcite and a trace of magnesium calcite (with its main diffraction peak at 28.7 nm) (Fig.4 A). After 3 cy
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