Bayerisches Geoinstitut, Universitätsstrasse 30, 95447 Bayreuth, Germany.
CNRS, Laboratoire de Geologie de Lyon, UMR 5276, Université Claude Bernard Lyon 1, Ecole Normale Supérieure de Lyon, 46, allée d'Italie, 69364 Lyon, France.
Nat Commun. 2017 Jun 7;8:15647. doi: 10.1038/ncomms15647.
In various shocked meteorites, low-pressure silica polymorph α-cristobalite is commonly found in close spatial relation with the densest known SiO polymorph seifertite, which is stable above ∼80 GPa. We demonstrate that under hydrostatic pressure α-cristobalite remains untransformed up to at least 15 GPa. In quasi-hydrostatic experiments, above 11 GPa cristobalite X-I forms-a monoclinic polymorph built out of silicon octahedra; the phase is not quenchable and back-transforms to α-cristobalite on decompression. There are no other known silica polymorphs, which transform to an octahedra-based structure at such low pressures upon compression at room temperature. Further compression in non-hydrostatic conditions of cristobalite X-I eventually leads to the formation of quenchable seifertite-like phase. Our results demonstrate that the presence of α-cristobalite in shocked meteorites or rocks does not exclude that materials experienced high pressure, nor is the presence of seifertite necessarily indicative of extremely high peak shock pressures.
在各种冲击陨石中，通常可以在与已知最密集的 SiO 多形体赛菲特石（稳定存在于 80GPa 以上）密切相关的空间关系中发现低压二氧化硅多形体 α-方石英。我们证明，在静水压力下，α-方石英至少在 15GPa 时仍未发生转变。在准静压实验中，在 11GPa 以上，方石英 X-I 形成了一种由硅八面体构成的单斜多形体；该相不可淬火，在减压时会回转变为α-方石英。在室温下压缩时，没有其他已知的二氧化硅多形体在如此低的压力下转变为基于八面体的结构。在非静水条件下进一步压缩方石英 X-I，最终会形成可淬火的赛菲特石状相。我们的结果表明，冲击陨石或岩石中方石英的存在并不排除材料经历了高压，而赛菲特石的存在也不一定表明存在极高的冲击压力峰值。
