Metamorphic garnet commonly contains needle‐like rutile inclusions as well as equant rutile inclusions that surround quartz inclusions and range in size from submicrometer to nanometer. Although the origin of these equant rutile inclusions, that is, exsolution or non‐exsolution, has important implications for petrological and tectonic processes, the crystallographic characteristics of these inclusions have rarely been studied because of the small sizes and analytical difficulties involved. Here, we report the crystallographic characteristics pertinent to the genetic origin of minute equant rutile inclusions in cloudy, nearly spherically shaped garnet domains with Ti‐depleted compositions surrounding quartz inclusions in ultrahigh‐pressure garnet from several diamondiferous Erzgebirge quartzofeldspathic gneissic rock samples. TEM analyses show that the equant rutile crystals in cloudy garnet domains are partially bounded by the low‐energy {100}_rt ± {110}_rt ± {101}_rt facets and have rather random crystallographic orientation relationships (CORs) with the garnet host, with preferential alignment of low‐energy lattice planes, for example, {100}_rt//{112}_grt, for some rutile crystals. Although the rather random CORs are unlikely to be attributed to solid‐state exsolution subjected to the stringent topotactic garnet lattice constraints, the characteristic subhedral {100}_rt ± {110}_rt ± {101}_rt crystal forms of rutile can be rationalized by a metasomatic dissolution‐reprecipitation mechanism via a fluid phase. In this scenario, the quartz+fluid inclusions in garnet were first subjected to decompression microcracking during rock exhumation, followed by dissolution of Ti‐bearing garnet matrix at the crack tips or along the crack surfaces and subsequent reprecipitation of rutile, apatite, gahnite, akdalaite, and Ti‐depleted garnet. The rapid coalescence between rutile and garnet crystals in fluid or direct attachment of rutile crystals onto the dissolving crack surfaces would then yield the rather random CORs as reported here. These results, along with previous work on rutile needles, indicate rather diverse genesis of rutile inclusions in various crystal forms, thus shedding light on the controversial exsolution origin for other inclusion suite/microstructure in minerals.

中文翻译：

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石榴石中金红石的溶解-再沉淀机理

变质石榴石通常包含针状金红石夹杂物以及等同的金红石夹杂物，它们围绕石英夹杂物，尺寸范围从亚微米到纳米。尽管这些相等的金红石夹杂物的起源（即析出或非析出）对岩石学和构造过程具有重要意义，但由于其尺寸小和分析困难，很少研究这些夹杂物的晶体学特征。在这里，我们报告了与多云，接近球形的石榴石域中的微小等量金红石夹杂物的成因有关的晶体学特征，这些贫富石榴石域中的Ti贫化成分围绕着来自几个含钻石的Erzgebirge石英长石质片麻质岩石样品的超高压石榴石中的石英夹杂物。_rt  ±{110} _rt  ±{101} _rt面，与石榴石主体具有相当随机的晶体取向关系（CORs），且优先选择低能晶格平面，例如{100} _rt // {112} _grt，用于一些金红石晶体。尽管相当随机的COR不太可能归因于严格的石榴石晶格约束所致的固态析出，但特征亚面{100} _rt  ±{110} _rt  ±{101} _rt金红石的晶体形式可以通过液相的交代溶解-再沉淀机理来合理化。在这种情况下，首先将石榴石中的石英+流体包裹体在岩石挖掘过程中进行减压微裂纹，然后在裂纹尖端或沿裂纹表面溶解含钛石榴石基体，然后使金红石，磷灰石，钠长石，akdalaite重新沉淀，以及贫钛的石榴石。金红石和石榴石晶体在流体中的快速聚结或金红石晶体直接附着在溶解的裂纹表面上会产生相当随机的COR，如此处报道。这些结果以及以前有关金红石针的研究表明，金红石内含物以各种晶体形式存在着多种多样的来源，