Extensive studies of the crystal-rich inclusions (CIs) hosted in minerals in pegmatite have resulted in substantially different models for the formation mechanism of the pegmatite. In order to evaluate these previously proposed formation mechanisms, the total homogenization processes of CIs hosted in spodumene from the Jiajika pegmatite deposit in Sichuan, China, were observed in situ under external H 2 O pressures in a new type of hydrothermal diamond-anvil cell (HDAC). The CIs in a spodumene chip were loaded in the sample chamber of HDAC with water, such that the CIs were under preset external H 2 O pressures during heating to avoid possible decrepitation. Our in situ observations showed that the crystals within the CIs were dissolved in carbonic-rich aqueous fluid during heating and that cristobalite was usually the first mineral being dissolved, followed by zabuyelite and silicate minerals until their total dissolution at temperatures between 500 and 720 ∘ C. These observations indicated that the minerals within the CIs were daughter minerals crystallized from an entrapped carbonate-and silica-rich aqueous solution and therefore provided useful information for evaluating the formation models of granitic pegmatites.

A new type of hydrothermal diamond-anvil cell (HDAC-VII) and its accompanied cooling system were designed. The design of HDAC-VII in which the three posts work simultaneously as guideposts and screw posts greatly shortened the horizontal size of HDAC compared with older types. It provides more open space and shorter distance to analyze and observe the sample chamber from side access. Moreover, four ports were used to connect the upper and lower spaces between windows and anvils, so mixed gas (Ar + H2) can pass through both of them. In the heating experiments, the mixed gas prevents diamond anvils and other parts from being oxidized as well as cooling the observing windows. Dry gas can be passed through those spaces during cooling, preventing condensation on the table faces of anvils and the observing windows. The cooling system can cool the sample chamber to −170 °C with an accuracy of ±0.5 °C by using a nitrogen stream cooled through a stainless steel coil immersed in a liquid nitrogen Dewar. The heating rates while reheating a frozen sample can be controlled to be 0.1 °C/min with a temperature fluctuation of 0.1 °C. These improvements extend the HDAC applications especially in low temperature conditions. For example, (1) we measured the salinities of NaCl–H2O loaded in the sample chamber, (2) we observed the ice I and VI-melting process and correspondingly calculated the density of water in the sample chamber, and (3) we performed lepidolite crystallization in an aqueous solution with 10 wt. % NaCl to simulate its natural forming conditions.