利用高壓實驗解開隕石中蘊藏的謎題

原文網址：www.sciencedaily.com/releases/2017/06/170607123736.htm

利用高壓實驗解開隕石中蘊藏的謎題

X光分析揭露二氧化矽礦物不為人知的性質

運用德國電子同步加速器研究所(DESY)的PETRA III光源和其他設施產生的X光，以拜羅伊特大學的Leonid Dubrovinsky為核心的研究團隊，解決了一項長久以來分析從月亮和火星來的隕石時所面臨的謎題。刊登在期刊《自然通訊》(Nature Communications)的這篇研究，可以解釋兩種不同的二氧化矽礦物為何能在隕石中共存，即便正常來說形成它們所需的條件大不相同。結論同樣也意謂之前對隕石形成環境的推測結果，必須要重新仔細審視才行。

科學家研究了一種稱作方矽石(cristobalite)的二氧化矽礦物。第一作者，拜羅伊特大學高溫高壓地科研究中心的Ana Černok表示：「在研究隕石之類從外星來的樣品時，科學家對此礦物特別感興趣，因為它們是二氧化矽礦物在地外物質中的主要型態。」她現在於英國公開大學進行研究。共同作者，里昂高等師範學校和國家科學研究中心的Razvan Caracas接著表示：「方矽石跟石英有相同的成分，但構造卻差異甚大。」

跟隨處可見的石英不同，由於方矽石僅能在特殊情況下於非常高的溫度形成，所以在地球表面相對來說很難找到方矽石。但是在月亮和火星來的隕石中，方矽石可說是相當常見。這些隕石是小行星撞擊到月亮或火星表面後濺射到太空，最終落到地球上的岩石。

驚人的是，研究人員發現火星和月球的隕石中，賽石英(seifertite)這種二氧化矽礦物會跟方矽石一起出現。Dubrovinsky和同事於20年前首度合成賽石英，它需要極高的壓力才能形成。Dubrovinsky強調：「由於方矽石和賽石英形成的溫度壓力條件十分不同，因此很難解釋為何能在同一塊隕石中找到它們。受到這項令人好奇的觀察啟發，20多年來有無數實驗和理論研究在探討方矽石於高壓下的行為，但問題依然沒有解決。」

利用(法國)格勒諾布爾的歐洲同步輻射裝置和DESY的PETRA III製造出來的強力X光，科學家現在可以前所未有地得到方矽石的構造在高達83 GPa(1 GPa=10億帕斯卡)，相當於大氣壓力82萬倍左右的壓力下會如何變化的影像。「研究顯示在各方向均等或是幾乎均等的情況下――我們稱之為靜水壓力(hydrostatic)或者準靜壓(quasi-hydrostatic)條件――壓縮方矽石的時候，它會呈現出稱作方矽石X-I的高壓相。」共同作者，DESY的Elena Bykova解釋。她任職於實驗進行的場所，PETRA III的極端條件光束線P02.2。「當壓力釋放後，此高壓相會回復成一般的方矽石。」

但是就像實驗顯示的，如果方矽石在科學家所稱的非靜壓(non-hydrostatic)條件下各方向並非均等壓縮，方矽石回復時就會出乎意料地變成一種跟賽石英很相似的構造。相較於石英平常轉變成賽石英需要的壓力，這種構造可以在低上許多的壓力下形成。Caracas表示：「運用從頭計算法證實了新的礦物相在高壓下具有動態穩定性。」此外，當壓力釋放後它還是能維持穩定。

Černok表示：「這項結果相當意外。我們的研究釐清了受到擠壓的方矽石如何能在比預期低上許多的壓力下轉變成賽石英。因此，同時含有賽石英跟方矽石的隕石，並不代表一定曾經歷經過相當巨大的撞擊事件。」在撞擊發生期間，衝擊波通過岩石內部時會形成非常複雜的應力狀態，甚至能讓物質處於靜水壓力和非靜壓壓縮的區域交互出現，使得同一塊隕石中可以形成不同類型的二氧化矽。

Dubrovinsky強調：「結果對於研究太陽系內部發生的撞擊作用有直接啟發。它們清楚證實方矽石和賽石英兩者，都不應該視為可靠跡象能指示出隕石曾歷經巨型撞擊產生的溫壓環境。」正如Dubrovinsky解釋的，這些觀察結果更常呈現出的是，同樣物質在靜水壓力和非靜壓條件下壓縮，會有天差地遠的反應。「對材料科學來說，我們的結果提出了要控制物質的性質還有另一種方法：除了溫度和壓力外，不同型態的應力或許也能讓固態物質呈現出完全不同的行為。」

High-pressure experiments solve meteorite mystery

X-ray analysis reveals unexpected behaviour of silica minerals

With high-pressure experiments at DESY's X-ray light source PETRA III and other facilities, a research team around Leonid Dubrovinsky from the University of Bayreuth has solved a long standing riddle in the analysis of meteorites from Moon and Mars. The study, published in the journal Nature Communications, can explain why different versions of silica can coexist in meteorites, although they normally require vastly different conditions to form. The results also mean that previous assessments of conditions at which meteorites have been formed have to be carefully re-considered.

The scientists investigated a silicon dioxide (SiO2) mineral that is called cristobalite. "This mineral is of particular interest when studying planetary samples, such as meteorites, because this is the predominant silica mineral in extra-terrestrial materials," explains first author Ana Černok from Bayerisches Geoinstitut (BGI) at University Bayreuth, who is now based at the Open University in the UK. "Cristobalite has the same chemical composition as quartz, but the structure is significantly different," adds co-author Razvan Caracas from CNRS, ENS de Lyon.

Different from ubiquitous quartz, cristobalite is relatively rare on Earth's surface, as it only forms at very high temperatures under special conditions. But it is quite common in meteorites from Moon and Mars. Ejected by asteroid impacts from the surface of Moon or Mars, these rocks finally fell to Earth.

Surprisingly, researchers have also found the silica mineral seifertite together with cristobalite in Martian and lunar meteorites. Seifertite was first synthesised by Dubrovinsky and colleagues 20 years ago and needs extremely high pressures to form. "Finding cristobalite and seifertite in the same grain of meteorite material is enigmatic, as they form under vastly different pressures and temperatures," underlines Dubrovinsky. "Triggered by this curious observation, the behaviour of cristobalite at high-pressures has been examined by numerous experimental and theoretical studies for more than two decades, but the puzzle could not be solved."

Using the intense X-rays from PETRA III at DESY and the European Synchrotron Radiation Facility ESRF in Grenoble (France), the scientists could now get unprecedented views at the structure of cristobalite under high pressures of up to 83 giga-pascals (GPa), which corresponds to roughly 820,000 times the atmospheric pressure. "The experiments showed that when cristobalite is compressed uniformly or almost uniformly -- or as we say, under hydrostatic or quasi-hydrostatic conditions -- it assumes a high-pressure phase labelled cristobalite X-I," explains DESY co-author Elena Bykova who works at the Extreme Conditions Beamline P02.2 at PETRA III, where the experiments took place. "This high-pressure phase reverts back to normal cristobalite when the pressure is released."

But if cristobalite is compressed unevenly under what scientists call non-hydrostatic conditions, it unexpectedly converts into a seifertite-like structure, as the experiments have now shown. This structure forms under significantly less pressure than necessary to form seifertite from ordinary silica. "The ab initio calculations confirm the dynamical stability of the new phase up to high pressures," says Caracas. Moreover it also remains stable when the pressure is released.

"This came as a surprise," says Černok. "Our study clarifies how squeezed cristobalite can transform into seifertite at much lower pressure than expected. Therefore, meteorites that contain seifertite associated with cristobalite have not necessarily experienced massive impacts." During an impact, the propagation of the shock wave through the rock can create very complex stress patterns even with intersecting areas of hydrostatically and non-hydrostatically compressed materials, so that different versions of silica can form in the same meteorite.

"These results have immediate implications for studying impact processes in the solar system," underlines Dubrovinsky. "They provide clear evidence that neither cristobalite nor seifertite should be considered as reliable tracers of the peak shock conditions experienced by meteorites." But the observations also show more generally that the same material can react very differently to hydrostatic and non-hydrostatic compression, as Dubrovinsky explains. "For materials sciences our results suggest an additional mechanism for the manipulation of the properties of materials: Apart from pressure and temperature, different forms of stress may lead to completely different behaviour of solid matter."

原始論文：Ana Černok, Katharina Marquardt, Razvan Caracas, Elena Bykova, Gerlinde Habler, Hanns-Peter Liermann, Michael Hanfland, Mohamed Mezouar, Ema Bobocioiu, Leonid Dubrovinsky. Compressional pathways of α-cristobalite, structure of cristobalite X-I, and towards the understanding of seifertite formation. Nature Communications, 2017; 8: 15647 DOI: 10.1038/ncomms15647

引用自：Deutsches Elektronen-Synchrotron DESY. "High-pressure experiments solve meteorite mystery: X-ray analysis reveals unexpected behaviour of silica minerals." ScienceDaily. ScienceDaily, 7 June 2017.