Hydrogen defects can strongly affect mechanical and chemical properties of feldspars. To get insight into the behavior of such defects, alkali feldspar and plagioclase of igneous origin were studied combining IR spectroscopy with heating experiments under well-controlled conditions. Near-infrared spectra show that OH groups are the predominant hydrous species in these feldspars but presence of minor amounts of molecular H2O cannot be excluded. Short-term annealing at 400–800 °C produces a small but significant irreversible change in the OH stretching vibration band which is attributed to relaxation of the feldspar structure. Polarized mid-infrared spectra of sanidine, adularia, and plagioclase recorded in situ at temperatures up to 600 °C show reversible shifts of maxima toward higher wavenumber and an overall decrease in integrated intensities. The pleochroic features of the OH vibration bands, i.e., the predominant orientation of OH dipoles along the crystallographic a axis in all feldspars and the additional band component perpendicular to the (010) plane in sanidine are still present in the high-temperature spectra. Different behavior during long-term annealing at high temperature was found for the alkali feldspars and the plagioclases. At 900–1000 °C, the Eifel sanidines rapidly lost about one quarter of the initial water content which is attributed to a weakly bound hydrogen species in the feldspar structure. The remaining hydrogen is very strongly bound and was still detectable in 0.7–0.9 mm thick sections after annealing for 108 days at 1000 °C in air dried by phosphorus pentoxide. In contrast, a 1-mm-thick section of plagioclase completely lost hydrogen during heating in air within 8 days at 1000 °C. After partial dehydration, the pleochroic behavior of the OH absorption bands of the feldspars was basically preserved except that the 3050 cm−1 band of the sanidine, oriented perpendicular to (010), becomes more pronounced than the 3400 cm−1 band, oriented parallel to the a direction. Annealing experiments at 1000 °C under controlled water pressures indicate equilibrium solubilities of several tens of ppm H2O in the plagioclases and more than 100 ppm H2O in the alkali feldspars already at 1 bar water pressure. The variation of the water content with H2O pressure and spectroscopic observations indicates that the water content in the feldspars is determined not only by the water pressure but also by already existing defects. Vacancies on alkali sites (VA1) may accommodate H2O molecules, possibly with subsequent hydrolysis of network bonds to minimize local stress. A likely explanation for the strongly bound hydrogen in the sanidine is a coupled substitution of H+ + Al3+ for Si4+ (AlOH defect) where the protons are located on interstitial sites. This incorporation model is supported by the complete recovery of the defects in H2O vapor after previous proton/alkali exchange in alkali chloride vapor at 1000 °C.

中文翻译：

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长石中的氢缺陷：缺陷特性及其对长石中水溶性的影响

氢缺陷会强烈影响长石的机械和化学性质。为了深入了解这些缺陷的行为，在良好控制的条件下结合红外光谱和加热实验研究了碱性长石和火成岩成因的斜长石。近红外光谱显示 OH 基团是这些长石中的主要含水物质，但不能排除少量分子 H2O 的存在。在 400-800°C 下进行短期退火会在 OH 伸缩振动带中产生微小但显着的不可逆变化，这是由于长石结构的松弛所致。在高达 600 °C 的温度下原位记录的sanidine、adularia 和斜长石的偏振中红外光谱显示最大值向更高波数的可逆位移和积分强度的整体下降。OH 振动带的多向色性特征，即 OH 偶极子在所有长石中沿晶体学 a 轴的主要取向和垂直于 sanidine 中的 (010) 平面的附加带分量仍然存在于高温光谱中。碱性长石和斜长石在高温长期退火过程中表现出不同的行为。在 900-1000 °C 时，Eifel sanidines 迅速失去了大约四分之一的初始含水量，这归因于长石结构中的弱结合氢物质。在 1000°C 下在五氧化二磷干燥的空气中退火 108 天后，剩余的氢结合力很强，并且在 0.7-0.9 毫米厚的切片中仍然可以检测到。相比之下，1 毫米厚的斜长石部分在 1000 °C 的空气中加热 8 天内完全失去了氢气。部分脱水后，长石的OH吸收带的多向色性基本保持不变，只是sanidine的3050 cm-1带，垂直于（010），比3400 cm-1带更明显，平行取向向 a 方向。在受控水压下在 1000 °C 下进行的退火实验表明，在 1 bar 水压下，斜长石中数十 ppm H2O 的平衡溶解度和碱性长石中超过 100 ppm H2O 的平衡溶解度。含水量随 H2O 压力和光谱观察的变化表明，长石中的含水量不仅取决于水压，还取决于已经存在的缺陷。碱性位点 (VA1) 上的空位可以容纳 H2O 分子，随后可能会发生网络键的水解以最小化局部应力。Sanidine 中强键合氢的一个可能解释是 H+ + Al3+ 对 Si4+（AlOH 缺陷）的耦合取代，其中质子位于间隙位点。在 1000 °C 的碱金属氯化物蒸汽中进行质子/碱交换后，H2O 蒸汽中的缺陷完全恢复，支持这种掺入模型。