遂安石的合成及其成因

Synthesis of Suanite and its Origin

遂安石是一种镁的硼酸盐矿物,自然界中较为少见。它于1953年由渡边武男在朝鲜遂安郡笏洞矿区首次发现,以后在苏联和我国也相继发现。它常与镁硼石(小藤石)、硼镁铁矿、硼镁石和粒硅镁石等矿物一起发育于矽卡岩型硼矿床中的变质白云岩中。遂安石之所以少见,是因其生成条件比较特殊。一般认为它是矽卡岩矿物,但典型的矽卡岩硼矿床中所含硼矿物往往是硼镁石和硼镁铁矿,而不是遂安石.

Suanite,a mineral of magnesium borate,rarely occurs in nature.We have studied the conditions for its formation and stability by laboratory synthesis and hydrothermal experiments.The experiments were carried out in cold-seal vessels at the temperature of 350-500 C and pressure of 200-1000 bars,with MgCO_(3)as a starting material and H3BO_(3),Na2B4O7 and H3BO_(3)+NaCl as reactive solutions.Suanite,kotoite,szaibelyite and ludwigite were formed under these conditions.The experimets show.(1)suanite occurs in a high-boron and magnesium,low-iron hydrothermal environment.It can be formed by reaction of MgCO_(3)with 1-3 M H3BO_(3)or Na2B4O7 solutions under temperature of 500-600 C and pressure of 200-1000 bars.In the case of reaction of MgCO_(3)with relatively diluted H3BO_(3)solution and under the same P-T conditions,the product is kotoite instead of suanite.(2)Suanite is formed and stable at relatively high temperature(500-600 C)and may change into szaibelyite by hydration when temperature drops below 450C.(3)Suanite is formed under a low CO_(2)partial pressure and a wide range of pH values and is stable when pH=5-9.From the results of the experiments,it is considered,that suanite may not be a typical skarn mineral.It seems most probable that suanite is formed by high-temperature metamorphism of sedimentary formations of hydrous magnesium borates and boron-bearing magnesium carbonates.The hydrous borate minerals,when undergo high-grade regional or contact metamorphism,would change into anhydrous borate minerals such as suanite and kotoife through dehydration.During the contact metamorphism of the magnesium-carbonate formation,boron form formation iself and from endogenic hydrothermal solutions may also react with carbonates to form suanite.Therefore,it’s reasonable that suanite is formed by hightemperature metamorphism of sediments.