Reply to Kroll and Schmid-Beurmann's comment on “Water decreases displacive phase transition temperature in alkali feldspar” by Liu et al. (2018)

Abstract

Abstract. 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 H2O 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 H2O 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 (Ab67Or31An2, Ab66Or31An2, and Ab65Or33An3) 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).