方石英高温高压相变的实验研究

High-pressure and high-temperature phase transformation of cristobalite at a pressure of up to 63 GPa

在1450℃高温下热处理天然的长白山硅藻土以获得含有一定量杂质元素的方石英,并以此为样品,利用对称型DAC装置在0—63 GPa压力范围对其开展高温高压相变的实验研究。衍射方法采用同步辐射能量色散X射线衍射(EDXD)技术。实验结果表明,方石英的高压相变序列较复杂,在12、24、33 GPa都有新的衍射线出现,在40GPa时仍存在明显的衍射线条而并未完全非晶化,并且为一种类似于α-PbO_2型结构的高压相。在此压力下用YLF激光器加温到大约1600K,降温后样品相变为类似于斯石英的结构。随后继续加压到62.9 GPa,样品未发生相变,并于此压力下给样品加温到大约1500 K,降温后其结构仍不发生变化。卸压至常压,仍为类似于斯石英的结构。

The sample of cristobalite including some impure elements was obtained by heating treatment of natural Changbaishan diatomite at 1450°C for 2h, and it was confirmed to be a single phase according to the powder X-ray diffraction and FTIR results. The high-pressure and high-temperature experiment of cristobalite was carried out at a pressure up to 63 GPa by symmetrical diamond anvil cell(DAC) apparatus and synchrotron radiation energy dispersive X-ray diffraction method at BSRF. The results indicate that cristobalite underwent serial phase transformations at up to 40 GPa, and at this pressure point, the new high-pressure phase of cristobalite is similar to the α-PbO_2 type structure except a diffraction line at about 0.3798 nm. The diffraction modes of a series of high-pressure metastable phases of cristobalite have the successive characteristic, which maybe reflect the fact that the original structure of cristobalite is favorable for the formation of the post-stishovite structure. The sample was heated by YLF laser to about 1600 K at 40 GPa. After heating, the sample was transformed to a new structure which was similar to the stishovite structure, and the experimental pressure was lowered to 34.8 GPa. Then the sample was pressed to 62.9 GPa, and its structure has no change. After heating to about 1500 K at this pressure, the sample has no structural transformation, and the pressure fell to 53.9 GPa. The sample still keeps the stishovite-like structure after the pressure unloaded. The experimental results maybe provide some useful hues to discuss the formation mechanism of natural SiO_2 post-stishovite phases.