Reply to Kroll and Schmid-Beurmann's comment on “Water decreases displacive phase transition temperature in alkali feldspar” by Liu et al. (2018)
<p>It has long been known that hydrogen impurities can be incorporated in the structure of nominally anhydrous minerals (NAMs) and substantially influence their physical properties. One of the geologically most prominent NAMs is feldspar. The hydrogen concentration in NAMs is usually expressed...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-05-01
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| Series: | European Journal of Mineralogy |
| Online Access: | https://ejm.copernicus.org/articles/32/305/2020/ejm-32-305-2020.pdf |
| Summary: | <p>It has long been known that hydrogen impurities can be incorporated in the
structure of nominally anhydrous minerals (NAMs) and substantially influence
their physical properties. One of the geologically most prominent NAMs is
feldspar. The hydrogen concentration in NAMs is usually expressed in parts per million of
water by weight (ppm <span class="inline-formula">H<sub>2</sub>O</span> wt.) In this paper, we use the term
“hydrogen” for uniformity, except when we use “water” for describing its
amount expressed as parts per million of <span class="inline-formula">H<sub>2</sub>O</span> by weight. In our article (Liu et al.,
2018), we carried out in situ high-temperature X-ray powder diffraction and
Raman spectroscopic studies on three natural anorthoclase samples with
similar Or (K-feldspar) contents (<span class="inline-formula">Ab<sub>67</sub>Or<sub>31</sub>An<sub>2</sub></span>,
<span class="inline-formula">Ab<sub>66</sub>Or<sub>31</sub>An<sub>2</sub></span>, and <span class="inline-formula">Ab<sub>65</sub>Or<sub>33</sub>An<sub>3</sub></span>) and Al–Si
disordering but contrasting water contents. The spectroscopic results
suggested that the displacive phase transition temperature is higher for the
nearly anhydrous anorthoclase sample than the anorthoclase samples with
about 200 ppm water, and we thus concluded that hydrogen is another factor
impacting the displacive phase transition temperature. We thank Kroll and
Schmid-Beurmann for pointing out the weakness in our interpretation
that hydrogen is a possible important factor (Kroll and Schmid-Beurmann,
2020). To clarify this issue, we conducted transmission electron microscopy
(TEM) experiments on the three samples to check texture effects. The TEM
studies indicated that the nearly anhydrous anorthoclase sample consists of
two feldspar phases, a K-poor and a K-rich one, and that the K-poor area may
be responsible for the higher displacive phase transition temperature.
According to the observation that the temperature of redistribution of
hydrogen is accordant with the displacive phase transition temperature, the
effect of hydrogen could not be ruled out. Based on these results, it can be
concluded that hydrogen may not be the sole possible factor, and it was a
proposition more than a definitive proof for the moment. Natural feldspars
are complex, and factors affecting displacive phase transitions are multiple
(e.g., Salje et al., 1991; Harrison and Salje, 1994; Hayward and Salje,
1996; Dobrovolsky et al., 2017). Therefore, to further investigate hydrogen
effects on displacive phase transition in feldspar, synthetic samples with
pure chemical compositions and hydrogen species are necessary. In the
following, we address each issue in the same order as in the comment by
Kroll and Schmidt-Beurmann (2020).</p> |
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| ISSN: | 0935-1221 1617-4011 |