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Pyrophyllite -- supporting role in Diamond Synthesis

March 15,2024

Pyrophyllite is a natural mineral - hydrous silicate. Its molecular formula is Al2O3·4SiO2·H2O, with a complex layered structure containing silicon-oxygen tetrahedra, and the interlayer is easy to slip. Its melting point under normal pressure exceeds 1670K. When the pressure is 5-6GPa, its melting point exceeds 2000℃. Under normal conditions, its resistivity is about 106~107Ω·m, and its resistivity decreases with increasing pressure and temperature. Under the pressure and temperature conditions for synthesizing diamond,  the resistivity decreases to 100Ω·m, but it still plays a good role in electrical insulation. Pyrophyllite has very low thermal conductivity, which does not change much with temperature and pressure. At a pressure of 2.5GPa, the internal friction coefficient of pyrophyllite is 0.25, and its shear strength increases with increasing compressive stress. Therefore, in terms of mechanical properties, pyrophyllite is a good sealing material.


In recent decades, almost all diamond manufacturers have used pyrophyllite as a sealing and pressure-transmitting medium. Pyrophyllite has many excellent properties that match well: good chemical stability, low thermal expansion, low thermal conductivity, high insulation, high melting point, high corrosion resistance, and low electrical conductivity. During the synthesis of diamond, pressure is transmitted from the anvil to the synthetic rod through pyrophyllite.

Pyrophyllite plays a very important role in diamond production as a main pressure-transmitting and sealing medium minerals in pyrophyllite

(1)The influence of mineral content in pyrophyllite on the synthesis of diamond The mineral composition of natural pyrophyllite is uneven, and the composition of pyrophyllite from different sources and different locations varies greatly, with different properties. Generally speaking, pyrophyllite with high SiO2 content is hard; pyrophyllite with high aluminum content is soft; when Fe2O3 content is high, the internal friction coefficient is high, which is beneficial for sealing; when H2O content is high, the texture is soft and the resilience is good. Synthetic diamond in China is mainly carried out on a cubic press, and pyrophyllite materials should be soft, so the SiO2 content should be low.

Pure pyrophyllite mineral aggregates are rare in nature, and are usually accompanied by minerals such as quartz, kaolinite, and gibbsite. If this pyrophyllite is directly used as a pressure transmission medium, it will lead to large fluctuations in the performance of diamond products and poor process reproducibility.


To address this issue, a method of powder pressing pyrophyllite blocks can be used. Mixed and crushed different types of natural pyrophyllite, then use its powder to press and mold it, which can make its pressure transmission performance more consistent.

This method greatly reduces the impact of mineral composition differences on diamond synthesis, making full use of pyrophyllite resources, providing good process repeatability and stabilizing product performance. China adopted the powder method to form pyrophyllite components early on, and currently almost all domestic synthetic diamond production companies use this process.
 When pressing powder blocks, special attention should be paid to using composite materials with different calcination temperatures, ensuring the sealing performance of pyrophyllite on the edges of the block, and ensuring the pressure transmission performance of pyrophyllite inside the block. In actual production, the calcination temperature should be adjusted according to specific circumstances, and cannot be generalized here. structural water and phase transition of pyrophyllite

(1)The influence of structural water and phase transition on pressure transmission, sealing, and diamond growth environment
The basic structural units of pyrophyllite are silicon-oxygen tetrahedrons and aluminum-oxygen octahedrons. The structural water in pyrophyllite is bonded to aluminum to form a "water-aluminum-oxygen layer" that is distributed between two silicon-oxygen tetrahedrons. As a solid pressure transmission medium, pyrophyllite undergoes a series of mineral phase transitions under high temperature and high pressure, resulting in a significant impact on its pressure transmission performance. For example, Beijing gray pyrophyllite begins to dehydrate above 770K and completely dehydrates at 1220K. When calcined above 1070K, it is accompanied by volume expansion. This is due to an increase in the lattice constant in a certain direction of the crystal, and the interlayer spacing of the sheet-like structure significantly increases. Pyrophyllite calcined at 1470K has decomposed into quartz + alumina + mullite. Under sufficient pressure (above 2.5GPa) and high temperature, pyrophyllite becomes coesite (SiO2) and lapis lazuli (Al2O3·SiO2), which is accompanied by a decrease in volume, resulting in a decrease in pressure in the high-pressure chamber. Due to the different varieties and types of pyrophyllite and a series of changes (structural water removal, phase transition, etc.) during pressure baking, the pressure transmission performance inevitably changes. Under untreated or low-temperature drying conditions, the gray color has the best pressure transmission performance, followed by white, spotted, and red colors.


In the process of synthesizing diamond, the temperature and pressure of the synthetic block of pyrophyllite are subject to certain gradients. On the cubic press, the requirements for the synthetic block of pyrophyllite are both pressure transmission and sealing: near the synthetic rod, pyrophyllite requires good pressure transmission performance, and pyrophyllite should have hydrostatic properties. The pressure difference inside the pyrophyllite block should be as small as possible, so pyrophyllite should have a small shear strength; far away from the synthetic rod, pyrophyllite requires good sealing performance to withstand large pressure gradients to maintain the pressure inside the cavity, which requires pyrophyllite to have a large shear strength. Pyrophyllite is a hydrous aluminosilicate, and structural water has a significant impact on the internal friction coefficient and shear strength of pyrophyllite. In production practice, the inner layer selects high-temperature roasted clinker, which reduces the pressure transmission performance, but avoids the evaporation of structural water in the raw material in the synthetic environment, which deteriorates the growth environment of diamond and reduces product quality. The outer layer should select raw materials that retain appropriate structural water to ensure that pyrophyllite has good plasticity, resilience, and appropriate internal friction. Therefore, the roasting temperature should consider the effects of pressure transmission and sealing as well as structural water.

The research on the rational utilization of pyrophyllite continues. Although it is only a supporting role in high-pressure synthesis, it plays an important role. With the diversification of synthetic products and the development of large-scale chambers, pyrophyllite will receive more and more attention in production.