Abstract
Feldspars are rock-forming minerals that make up most of the Earth’s crust. Along the mantle geotherm, feldspars are stable at pressures up to 3 GPa and may persist metastably at higher pressures under cold conditions. Previous structural studies of feldspars are limited to ~10 GPa, and have shown that the dominant mechanism of pressure-induced deformation is the tilting of AlO4 and SiO4 tetrahedra in a tetrahedral framework. Herein, based on results of in situ single-crystal X-ray diffraction studies up to 27 GPa, we report the discovery of new high-pressure polymorphs of the feldspars anorthite (CaSi2Al2O8), albite (NaAlSi3O8), and microcline (KAlSi3O8). The phase transitions are induced by severe tetrahedral distortions, resulting in an increase in the Al and/or Si coordination number. High-pressure phases derived from feldspars could persist at depths corresponding to the Earth upper mantle and could possibly influence the dynamics and fate of cold subducting slabs.
Highlights
Feldspars are rock-forming minerals that make up most of the Earth’s crust
The phase transition is accompanied by drastic modifications of the tetrahedral framework, mostly pronounced in an increase in the Al coordination number
The well-established phase diagrams of anorthite, albite and Kfeldspar suggest that these compounds decompose along normal mantle geotherm at pressures below ~3 GPa according to the following reactions:
Summary
Feldspars are rock-forming minerals that make up most of the Earth’s crust. Along the mantle geotherm, feldspars are stable at pressures up to 3 GPa and may persist metastably at higher pressures under cold conditions. Based on results of in situ single-crystal X-ray diffraction studies up to 27 GPa, we report the discovery of new high-pressure polymorphs of the feldspars anorthite (CaSi2Al2O8), albite (NaAlSi3O8), and microcline (KAlSi3O8). Plagioclases compose most of the Moon’s crust and have been detected on the surfaces of Mars[4,5], Venus[6] and Mercury[7], as well as in chondrites[8] Such high geological relevance of feldspars has led to numerous experimental and theoretical studies on high-pressure (P)-hightemperature (T) phase relations[9,10,11], elastic[12,13] and rheological properties[14], and the amorphization mechanism[15]. We propose that dense feldspar phases can withstand deep subduction along (ultra)cold subduction zones and influence the dynamics and fate of descending slabs
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