Geochemical Journal, Vol. 13, pp. 181 to 185, 1979 181

NOTE

Magnesian calcite synthesis from calcium bicarbonate solution containing magnesium and barium ions

YASUSHI KITANO', AKIRA TOKUYAMA2 and TAKESHI ARAKAKI'

Water Research Institute, Nagoya University, Chikusa-ku, Nagoya', and Department of Chemistry, Faculty of Science & Engineering, Ryukyu University, Naha, Okinawa2, Japan

(Received September 16, 1978; accepted in revised form June 4, 1979)

The present authors have studied the influence of barium and magnesium ions, which are contained together in a calcium bicarbonate parent solution, on the crystal form of carbonate formed from the parent solution. Magnesium ions in a parent solution inhibit calcite formation and favor aragonite formation, whereas barium ions inhibit aragonite formation and favor calcite formation. The degree of these influences is affected by the concentrations of these ions in the parent solution. When the influence of barium ions is stronger than that of magnesium ions, magnesian calcite is formed and the MgCO3 content of the magnesian calcite increases with increasing concentration of magnesium ions in a parent solution. From the calcium bicarbonate solutions containing both more than 35ppm of barium ions and 1,200ppm of magnesium ions monohydrocalcite is formed. Monohydrocalcite does not incorporate both MgCO3 and BaCO3 in its crystal lattice. The distribution coefficients of magnesium and barium have been measured when magnesian calcite is precipitated. The distribution coefficient of magnesium or barium for magnesian calcite tends to increase with increasing barium carbonate content or magnesium carbonate content of the formed magnesian calcite, respectively.

INTRODUCTION between calcite and magnesite or dolomite (CRAVE, 1952; GOLDSMITHet al., 1955; KITANO The skeletal parts of calcareous marine and FURUTSU, 1959; KITANO and KANAMORI, organisms are composed of the minerals, calcite, 1966). And magnesian calcite and aragonite aragonite and magnesian calcite (CHAVE,1952, are unstable, relative to Mg-poor calcite at 1954; LOWENSTAM,1954; KITANOand KANAMORI, ordinary temperature and pressure (GARRELSet 1966; KITANOet al., 1969). The study on the al., 1960; CHAVEet al., 1962; JANSEN and distribution of trace metals for these minerals KITANO,1963; PLUMMERand MACKENZIE,1974; is important for the understanding of the geo DE BOER, 1977). There are few reports on chemical behaviors of trace metals on the earth's the measurement of the distribution coefficients surface. And a number of studies have been of chemical elements for magnesian calcite. For carried out on the measurement of the distribu the partition study, it is necessary to synthesize tion coefficients of trace metals between solu magnesian calcite from aquatic solution at tion and carbonate precipitate; Sr (HOLLANDet normal temperature and pressure. al., 1963, 1964; B ODINEet al., 1965; KINSMAN Laboratory syntheses of magnesian calcite and HOLLAND,1969; KITANOet al., 1971,1973, were accomplished at high temperatures and KATZ et al., 1972), Zn (TSUSUEand HOLLAND, pressures (HARKERand TUTTLE,1955; GRAF and 1966; CROCKETand WINCHESTER,1966; KITANO GOLDSMITH,1956; GOLDSMITHand HEARD, 1961), et al., 1968), Cu (KITANO et al., 1968), U02 and recently at normal temperature and pressure (KITANOand OOMORI,1971), Mn (BODINEet al., from aqueous solutions (KITANOand KANAMORI, 1965; RAISWELLand BRIMBLECOMBE,1977), Ba 1966; GLOVERand SIPPLE, 1966; KITANOet al., (KITANOet al., 1971), Mg (KITANOand KANA 1976; OHDE and KITANO, 1978). A parent MORI)1966; KATZ, 1973; KITANOet al., 1976), solution must contain magnesium, calcium and Cd (KITANOet al., 1978). Magnesium carbonate carbonate ions for the precipitation of magnesian in skeletal calcite is present as a solid solution calcite having a calcitic lattice configuration. 182 Y. KITANO et al.

KITANO(1962) showed that magnesium ions in The release of carbon dioxide gas from these a parent solution favor the precipitation of solutions was followed by carbonate precipita aragonite, which does not capture magnesium tion. During the lapse of time in the process in its lattice. The presence of magnesium ions of carbonate formation, the changes in pH value in a parent solution inhibits magnesian calcite and the concentrations of calcium, magnesium very strongly. KITANO(1962) also reported and barium ions in a parent solution were mea that barium ions in a parent solution favor the sured. And the values of distribution coefficient precipitation of calcite, which does not capture were calculated with Doener Hoskin's equation. barium in its lattice. Thus when sufficient The magnesium and barium contents of the amounts of barium ions are contained in parent precipitate were determined and the crystal solutions containing magnesium ions even in form was identified. large amounts, magnesian calcite formation is expected from the solutions. The synthesis RESULTSAND DISCUSSION experiment based on this idea was tried previous ly, but magnesian calcite was not formed Effect of magnesium and barium ions in parent (KITANOet al., 1976). In order to disclose the solution on crystal form of precipitate Figure reason, detailed experiments on the crystal for 1 shows the relation between the initial con mation of carbonates have been made, and it centrations of barium and magnesium ions in has been confirmed that magnesian calcite is calcium bicarbonate parent solutions and the formed from calcium bicarbonate solutions con crystal forms of carbonates formed from the taining certain amounts of both barium and parent solutions at 25±2°C. The presence of magnesium ions. magnesium ions in the parent solution inhibits the precipitation of calcite and favors the pre EXPERIMENTAL cipitation of aragonite. The presence of barium ions inhibits the precipitation of aragonite and Procedure for carbonate formation to study favors the precipitation of calcite. Figure 1 effects of magnesium and barium ions in parent indicates the chemical conditions of a parent solution on crystal form of carbonate precipi solution for the formation of carbonate minerals. tate Magnesium chloride and barium chlo From Fig. 1, it is seen that magnesian calcite ride in various proportions were placed in is formed, when the concentration ratio of beakers (51) containing calcium bicarbonate barium ions to magnesium ions is larger than solution (Ca', 3 20 3 70 ppm). Carbon dioxide about 1/25 in an original solution. Figure 2 gas gradually escaped from the parent solution, shows the magnesium content of the calcites and carbonate was precipitated. The formed formed from calcium bicarbonate solutions con carbonate precipitate was filtered off and washed taining both magnesium and barium ions against with distilled water until the precipitate was free from the parent solution. After it had been dried in an air bath at 60°C, a fraction of /CaC03• H20or Mg -Calcite the precipitate was used for the determination of magnesium and barium contents, while C °C CaC03 •H20 another fraction. was used for identification of A0 Mg Calcite the crystal form by X-ray diffractometer and Ca C0 N 3 3•H0, Ar-~ also optical microscope. Calcium and mag 0 -t gonite nesium in the formed carbonate were deter a mined by atomic absorption spectrometer, and 20 /tea~/ E r~ Aragonite barium by flame emission spectrometer after a 10 calcium and magnesium ions had been removed rp with ion exchange resin. m 0 500 1000 1500 2000 Procedure for measurement of distribution coef Mg 2'(ppm) In parent solution ficien t Calcium bicarbonate solutions con taining magnesium ions (0 1,200ppm) and Fig. 1. Crystal forms of carbonates formed from cal also barium ions (0 50 ppm) were prepared. cium bicarbonate solutions (Ca 2'V 320 370ppm) con The solutions in beakers (51) were stirred fairly taining both magnesium (Mg'. 200 2,000ppm) and vigorously with a magnetic stirrer at 25±2°C. barium ions (Ba2+, 1.0 SOppm).

a Magnesian calcite synthesis 183

b 0 e 0.03 v 9

0 4 0.02 .u \V O O O O A O 00 0 U c O /.'W O 10 0 44 0 0.01 cr 0 a 0 u n O O

a monohydrocalcite 0 0.5 1.0 1.5 2.0

0 500 1000 1500 allors B°CO, content of Mg-Calcite (mol %) • Mg2 (ppm) in parent solution Fig. 3. Values of distributioncoefficient of magnesium Fig. 2. MgCO3 contents of calcites formed from calcium between calcite and calcium bicarbonateparent solution bicarbonate solutions (Ca 2+P320 370ppm) containing containingmagnesium (Mg2± 0 800ppm) and barium both magnesium (Mg2+, 300 2, 000ppm) and barium (Ba2+,0 40ppm) ions, against barium carbonate con ions (Ba2+, 10 50ppm). tent of carbonate precipitate.

the original concentration of magnesium ions in the calcium bicarbonate parent solutions. The 0.4 0 O formed carbonates in Fig. 2 have all calcite type O 0 crystal form. And the figure indicates that the O magnesium carbonate content of the formed O 0 calcites increases with increasing concentration 0 0 of magnesium ions within 1,000ppm in the parent solution. But in the parent solution con taining more than 1, 200 ppm and 3 5 ppm of magnesium and barium ions respectively, the magnesium carbonate content of the formed calcites became extremely small (see Fig. 2), 0 2.5 5.0 7.5 10.0

because the formed calcites were not magnesian M9CO3 content of Mg -Calcite (mol % ) calcite but monohydrocalcite. Monohydrocal cite does not contain magnesium in its lattice as Fig. 4. Values of distribution coefficient of barium aragonite. between calcite and calcium bicarbonate parent solu The formation of monohydrocalcite, mag tion containing magnesium (Mg2+,0 800ppm) and nesian calcite or aragonite and also the mag barium(Ba2+, 0 40ppm) ions, against magnesiumcar nesium carbonate content of formed carbonates bonate content of carbonate precipitate. in the solution system are governed by the con centrations of magnesium and barium ions in parent solutions. of trace metal (Me) and calcium in carbonate precipitate respectively and aMe2+and aca+ de Measurement of distribution coefficients of note the activities of metal ions (Me 2+) and cal barium and magnesium between carbonate pre cium ions in a parent solution respectively. It cipitate and solution The distribution coef is noted that the ionic radii of magnesium and ficients of magnesium (DMg) and barium (DBa ) barium are smaller and larger, respectively, than between solution and calcite have been calculat that of calcium. ed in the following system: Ca(HCO3)2 (Ca2+, Figure 3 shows a plot of DMg against the 320 370 ppm) + MgC12 (Mg', 0 800 ppm) barium carbonate content of formed calcites. + BaC12(Ba2+, 0 40ppm) -* Calcite (at 25± The figure indicates that the value of Dig tends 2°C). DMe is defined as (XMe/Xca)/(aMe2+/aca2+), to increase slightly with increasing barium car where XMe and Xca denote the molar fractions bonate content of calcites. Figure 4 shows a

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