New insights in the mechanisms of the reaction 3.65 Å phase = clinoenstatite + water down to nanoscales
<p>The reaction of 3.65 Å phase <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo><</mo><mo>=...
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| Autores principales: | , , , |
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| Formato: | article |
| Lenguaje: | EN |
| Publicado: |
Copernicus Publications
2021
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| Materias: | |
| Acceso en línea: | https://doaj.org/article/20178e10140f49ab9085dbfea6c774ce |
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| Sumario: | <p>The reaction of 3.65 Å phase <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo><</mo><mo>=</mo><mo>></mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="9pt" class="svg-formula" dspmath="mathimg" md5hash="4235923c16e433756c5ed2e06a091d75"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-33-675-2021-ie00001.svg" width="25pt" height="9pt" src="ejm-33-675-2021-ie00001.png"/></svg:svg></span></span> clinoenstatite <span class="inline-formula">+</span>
water was investigated in five experiments at 10 GPa, 470–600 <span class="inline-formula"><sup>∘</sup></span>C, using a rotating multi-anvil press. Under these <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>P</mi><mo>/</mo><mi>T</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="20pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="5f12af409049f834e7125daa3b34b4e2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-33-675-2021-ie00002.svg" width="20pt" height="14pt" src="ejm-33-675-2021-ie00002.png"/></svg:svg></span></span> conditions, clinoenstatite exists in its high-pressure modification, which, however, is not quenchable to ambient conditions but transforms back to low-pressure clinoenstatite. The quenched run products were characterized by electron microprobe analyses (EMPA), powder X-ray diffraction (XRD), Raman spectroscopy and by high-resolution transmission electron microscopy (HRTEM) on focused ion beam (FIB)-cut foils. We
bracketed the reaction in the <span class="inline-formula"><i>T</i></span> range 470 to 510 <span class="inline-formula"><sup>∘</sup></span>C (at 10 GPa). The hydration of clinoenstatite to the 3.65 Å phase at 470 <span class="inline-formula"><sup>∘</sup></span>C was very sluggish and incomplete even after 96 h. Clinoenstatites range in size from less than 1 to up to 50 <span class="inline-formula">µm</span>. Usually clinoenstatite has a very small grain size and shows many cracks. In sub-micron-sized broken clinoenstatite, an amorphous phase (<span class="inline-formula">0.91Mg:1.04Si</span>, with about 20 wt % H<span class="inline-formula"><sub>2</sub></span>O) was observed, which further transformed with increasing reaction time into the 3.65 Å phase (<span class="inline-formula">1Mg:1Si</span>, with 34 wt % H<span class="inline-formula"><sub>2</sub></span>O). Thus, the sub-micron-sized fractured clinoenstatite transformed via an amorphous water-bearing precursor phase to the 3.65 Å phase. The dehydration to clinoenstatite was faster but still incomplete after 72 h at 600 <span class="inline-formula"><sup>∘</sup></span>C. From the backscattered electron images of the recovered sample of the dehydration experiment, it is obvious that there is a
high porosity due to dehydration of the 3.65 Å phase. Again, the grain
size of clinoenstatite ranges from less than 1 up to 50 <span class="inline-formula">µm</span>.
There are still some clinoenstatite crystals from the starting material
present, which can clearly be distinguished from newly formed sub-micron-sized clinoenstatite. Additionally, we observe a water-rich
crystalline phase, which does not represent the 3.65 Å phase. Its Raman spectra show the double peaks around 700 and 1000 cm<span class="inline-formula"><sup>−1</sup></span> characteristic for enstatite and strong water bands at 3700 and 3680 cm<span class="inline-formula"><sup>−1</sup></span>. The <span class="inline-formula">Mg:Si</span> ratio of <span class="inline-formula">0.90:1.04</span> was determined by EMPA, totalling to 81 wt %, in accordance with its high water content. Diffraction patterns from high-resolution images (fast Fourier transform – FFT) are in agreement with an orthoenstatite crystal structure (Pbca).</p>
<p>The surprising observation of this study is that, in both directions of the
investigated simple reaction, additional metastable phases occur which are
amorphous in the hydration and crystalline in the dehydration reaction. Both additional phases are water rich and slightly deviate in composition from the stable products 3.65 Å phase and clinoenstatite, respectively. Thus, as a general remark, conventional investigations on reaction progress should be complemented by nanoscale investigations of the experimental products because these might reveal unpredicted findings relevant for the understanding of mantle processes.</p>
<p>The extreme reduction in grain size observed in the dehydration experiments
due to the formation of nanocrystalline clinoenstatite rather than the
slowly released fluids might cause mechanical instabilities in the
Earth's mantle and, finally, induce earthquakes.</p> |
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