An experimental study on K and Na incorporation in dravitic tourmaline and insight into the origin of diamondiferous tourmaline from the Kokchetav Massif, Kazakhstan
An experimental study on K and Na incorporation in dravitic tourmaline and insight into the origin of diamondiferous tourmaline from the Kokchetav Massif, Kazakhstan
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DOI：
10.1007/s00410-015-1116-9
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发表时间：
2015-02
影响因子：
3.5
通讯作者：
Eleanor J. Berryman;B. Wunder;R. Wirth;D. Rhede;G. Schettler;G. Franz;W. Heinrich
中科院分区：
文献类型：
--
作者：
Eleanor J. Berryman;B. Wunder;R. Wirth;D. Rhede;G. Schettler;G. Franz;W. Heinrich
在500 ~ 700℃、0.2 ~ 4.0 GPa的条件下，以氧化物混合物和过量流体为原料，在MgO-Al2O3-B2O3-SiO2-KCl-NaCl-H2O体系中合成了电气石，考察了压力、温度和流体组成对电气石中Na和K相对掺入量的影响。K在x位点的掺入随着压力、温度和KCl浓度的增加而增加；在4.78mKCl， Na-free流体中，在4.0 GPa， 700°C条件下合成的K-驱动石中，最大可掺入0.71 K pfu（留下0.29个x -空位pfu）。相反，钠的掺入主要取决于流体成分，而不是压力或温度；以3.87mNaCl和1.08mKCl为原料，在温度为0.4 GPa和700℃的条件下合成了Na含量最高的磁石，Na含量为1.00 Na pfu。所有合成的晶体均呈带状分布，含钠、含钾体系的主要固溶体为镁-辉石岩[□（Mg2Al）Al6Si6O18(BO3)3(OH)3OH]和吸石[NaMg3Al6Si6O18(BO3)3(OH)3]，吸石成分随流体中Na浓度的增加而增加。在含钾无钠体系中，主要固溶体为镁辉石岩和钾吸石[KMg3Al6Si6O18(BO3)3(OH)3(OH)]，钾吸石组分随压力、温度和流体中K浓度的增加而增加。随着K的掺入，电气石的单位胞体积从1555.1(3)增加到1588.1(2)Å3，反映了较大的K+离子的掺入。通过与哈萨克斯坦Kokchetav地块超高压岩石中maruyamaite（以钾为主的电气石）的成分对比，表明后者形成于靠近金刚石稳定场的超高压条件下的富钾、贫钠环境中。
Tourmaline was synthesized in the system MgO–Al2O3–B2O3–SiO2–KCl–NaCl–H2O from an oxide mixture and excess fluid at 500–700 °C and 0.2–4.0 GPa to investigate the effect of pressure, temperature, and fluid composition on the relative incorporation of Na and K in dravitic tourmaline. Incorporation of K at the X-site increases with pressure, temperature, and KCl concentration; a maximum of 0.71 K pfu (leaving 0.29 X-vacant sites pfu) was incorporated into K-dravite synthesized at 4.0 GPa, 700 °C from a 4.78mKCl, Na-free fluid. In contrast, Na incorporation depends predominately on fluid composition, rather than pressure or temperature; dravite with the highest Na content of 1.00 Na pfu was synthesized at 0.4 GPa and 700 °C from a 3.87mNaCl and 1.08mKCl fluid. All synthesized crystals are zoned, and the dominant solid solution in the Na- and K-bearing system is between magnesio-foitite [□(Mg2Al)Al6Si6O18(BO3)3(OH)3OH] and dravite [NaMg3Al6Si6O18(BO3)3(OH)3(OH)], with the dravitic component increasing with the concentration of Na in the fluid. In the K-bearing, Na-free system, the dominant solid solution is between magnesio-foitite and K-dravite [KMg3Al6Si6O18(BO3)3(OH)3(OH)], with the K-dravitic component increasing with pressure, temperature, and the concentration of K in the fluid. The unit-cell volume of tourmaline increases with K incorporation from 1555.1(3) to 1588.1(2) Å3, reflecting the incorporation of the relatively large K+ion. Comparison of our results to the compositional data for maruyamaite (K-dominant tourmaline) from the ultrahigh-pressure rocks of the Kokchetav Massif in Kazakhstan suggests that the latter was formed in a K-rich, Na-poor environment at ultrahigh-pressure conditions near the diamond-stability field.