The supracrustal metamorphic rocks of the Dora-Maira Massif, western Alps, have been intensively studied. Certain ultra-high-pressure lithologies contain coesite and nearly end-member composition pyrope, Mg 3 Al 2 Si 3 O 12 , making this locality petrologically and mineralogically unique. Structural OH - , loosely termed “water”, in pyrope crystals of different composition has been investigated numerous times, using different experimental techniques, by various researchers. However, it is not clear where the minor OH - is located in them. IR single-crystal spectra of two pyropes of composition {Mg 2.79 ,Fe 2+ 0.15 ,Ca 0.04 } Σ2.98 [Al] 2.02 (Si) 2.99 O 12 and {Mg 2.90 ,Fe 2+ 0.04 ,Ca 0.02 } Σ2.96 [Al] 2.03 (Si) 3 O 12 were recorded at room temperature (RT) and 80 K. The spectra show five distinct OH - bands located above 3600 cm -1 at RT and seven narrow bands at 80 K and additional fine structure. The spectra were curve fit and the OH - stretching modes analyzed and assigned. It is argued that OH - is located in microscopic- and nano-size Ca 3 Al 2 H 12 O 12 -like clusters. The basic substitution mechanism is the hydrogarnet one, where (H 4 O 4 ) 4- ⇔ (SiO 4 ) 4- , and various local configurations containing different numbers of (H 4 O 4 ) 4- groups define the cluster type. The amounts of H 2 O range between 5 and 100 ppm by weight, depending on the IR calibration adopted, and are variable among crystals. Hydrogrossular-like clusters are also present in a synthetic pyrope with a minor Ca content grown hydrothermally at 900 °C and 20 kbar, as based on its IR spectra at RT and 80 K. Experiment and nature are in agreement, and OH - groups are partitioned into various barely nano-size hydrogrossular-like clusters. This proposal is new and significant mineralogical, petrological, and geochemical implications result. Ca and proton ordering occur. Hypothetical “defect” and/or coupled-substitution mechanisms to account for structural OH - are not needed to interpret experimental results. OH - incorporation in pyrope of different generations at Dora-Maira is discussed and OH - could potentially be used as an indicator of changing $$ P_{{{\text{H}}_{{\text{2}}} {\text{O}}}} {\text{(}}a_{{{\text{H}}_{{\text{2}}} {\text{O}}}} {\text{) - }}T $$ P H 2 O ( a H 2 O ) - T conditions in a metamorphic cycle. Published experimental hydration, dehydroxylation, and hydrogen diffusion results on Dora-Maira pyropes can now be interpreted atomistically.

中文翻译：

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来自阿尔卑斯山西部 Dora-Maira 地块的超高压岩石中的镁橄榄石晶体中的微米和纳米尺寸的类水藻类簇

对阿尔卑斯山西部多拉-迈拉地块的壳上变质岩进行了深入研究。某些超高压岩性包含柯石英和近端元组成的镁铝榴石、Mg 3 Al 2 Si 3 O 12 ，使该地区在岩石学和矿物学上独一无二。结构 OH - ，松散地称为“水”，在不同成分的榴石晶体中，不同的研究人员使用不同的实验技术进行了多次研究。但是，尚不清楚次要 OH - 在其中的位置。两种成分为 {Mg 2.79 ,Fe 2+ 0.15 ,Ca 0.04 } Σ2.98 [Al] 2.02 (Si) 2.99 O 12 和 {Mg 2.90 ,Fe 2+ 0.04 ,Ca 0.02 } Σ2 的红外单晶光谱。在室温 (RT) 和 80 K 下记录了 96 [Al] 2.03 (Si) 3 O 12。光谱显示在室温下位于 3600 cm -1 以上的五个不同的 OH - 带和在 80 K 下的七个窄带和额外的精细结构。光谱是曲线拟合并且分析和分配OH - 拉伸模式。有人认为，OH - 位于微观和纳米尺寸的 Ca 3 Al 2 H 12 O 12 类簇中。基本的取代机制是水石榴石，其中 (H 4 O 4 ) 4- ⇔ (SiO 4 ) 4- 和包含不同数量的 (H 4 O 4 ) 4- 基团的各种局部构型定义了簇类型。H 2 O 的量按重量计在 5 到 100 ppm 之间，取决于采用的 IR 校准，并且在晶体之间是可变的。根据其在 RT 和 80 K 下的红外光谱，在 900 °C 和 20 kbar 下通过水热法生长的具有少量 Ca 含量的合成榴石中也存在类似 Hydrogrossular 的簇。实验和性质是一致的，OH - 基团被分成各种几乎纳米级的类似水草的簇。这个提议是新的和重要的矿物学、岩石学和地球化学意义的结果。Ca 和质子排序发生。解释实验结果不需要假设的“缺陷”和/或耦合替代机制来解释结构 OH。OH - 讨论了 Dora-Maira 不同世代的烟石中的掺入，并且 OH - 可能被用作变化的指标 $$ P_{{{\text{H}}_{{\text{2}}} {\ text{O}}}} {\text{(}}a_{{{\text{H}}_{{\text{2}}} {\text{O}}}} {\text{) - } }T $$ PH 2 O ( a H 2 O ) - 变质循环中的 T 条件。已发表实验水合、脱羟基、