Mineral inclusions in garnet: nanostructure-texture-genesis

Mineral inclusions embedded in larger host crystals can provide important information about the evolution of crystalline rocks or similar synthetic materials. Microstructural, crystallographic or compositional features like i) the inclusion distribution in the host crystal, ii) preferred shape orientation of inclusions, iii) the structure of the interface between inclusions and host crystal, iv) preferred crystallographic orientation of inclusion sets with respect to the host crystal and/or v) variations in the chemical composition of the involved minerals and their interfaces are used to infer the ambient conditions of inclusion formation. However, at present there are no unique criteria available that allow unequivocal determination of how inclusions formed. Furthermore, the information available from characterization of mineral inclusion host systems may differ when observing at different scales. By collaboration of researchers from the University of Vienna (Austria) and the Jožef Stefan Institute in Ljubljana (Slovenia) we intend to investigate mineral inclusions in garnet single crystals with various analytical methods that provide complementary information at different scales (millimeter, micrometer, nanometer and atomic scale). Different analytical instruments are required to cover this range of observation scales, and each method has certain strengths and weaknesses. Approaches working at millimeter to micrometer-scale can analyze sufficiently large sample domains to yield statistically representative information. On the other hand, time- and cost-intensive analyses at atomic scale resolution provide details of atomic arrangements of selected crystal domains, but only document very small sample portions that might not be representative. The combination of the different approaches therefore represents a promising way to analyze statistically representative features in sufficient detail. We aim to identify the microstructural, crystallographic and compositional footprint of certain inclusion formation mechanisms like overgrowth, intergrowth or exsolution from the host crystal. Results are expected to provide tools for identifying the mechanisms and conditions of inclusion formation, which is of interest for geoscientists who aim at deciphering the past rock evolution, and material scientists who aim at designing material with defined properties. The results are expected to enhance understanding of the chemical and mechanical behavior of polycrystalline materials under changing ambient conditions.

Characteristic microstructures of mineral inclusions in garnet host crystals can provide important information on the histories of rocks. To read this record, we need to know, which parameters control the formation of inclusion features, and whether the inclusions formed before, during or after host crystallization. Research in this project correlated geometric, crystallographic and compositional features of mineral inclusions and their garnet host, in order to refer the characteristics to certain conditions and mechanisms of inclusion formation. Comparison of different domains within single garnet crystals, and data from three different rock samples yielded systematic differences and similarities. A pristine pegmatoid from the Bohemian Massif (Gars am Kamp, AT), a metapegmatite from the Koralpe (Wirtbartl, AT) and a metapelitic gneiss from the Greek Rhodopes (Xianthi, GR) were investigated using various analytical methods for material characterization at different length scales, ranging from hand specimen (dm-mm) via single crystals (mm-nm) to single atoms (Å). In a correlated approach, we investigated the spatial distribution of the inclusion phases within the host crystals, the nature of their boundaries, the sizes, geometries and shape preferred orientations (SPOs) of needle-shaped inclusions, the crystal orientation relationships (CORs) between inclusions and their host, as well as compositional variations within the host crystals. The correlated datasets allowed identification of crystallographic, compositional and kinetic parameters that influence the formation of inclusion microstructures relating to particular relative timing relationships of the crystallization of inclusions and their host phase. The atomic configuration of the garnet growth facet can have a selective effect on the mineral phases that form by co-growth with garnet. Different growth sectors of a pegmatoid garnet show different predominant inclusion phases (phosphates in {110}, Ti-oxide in {112} garnet sectors) and different SPO-/COR-characteristics of needle-shaped Ti-oxide inclusions, highlighting the need to consider the particular growth facet of garnet when interpreting inclusion microstructures. On the other hand, when inclusions precipitate within a pre-existing garnet, all garnet sectors are expected to show the same SPO-and COR characteristics. Based on SPO frequencies of needle-shaped Ti-oxide inclusions in garnet, we can distinguish, whether inclusions formed during or after host crystallization, independent of a magmatic or metamorphic origin. Furthermore, compositional factors influence microstructure formation in the studied pegmatitic samples, which show similarities and differences. Differing inclusion characteristics in different growth zones within one sector are ascribed to changes in melt composition (P, OH, Si, Na) by fractional crystallization, corresponding to changes in size, geometry, SPO- and COR-characteristics of Ti-oxide inclusions. The chemical factors interact with kinetic factors, as changes in melt properties affect the rates of chemical diffusion, and the relative rates of nucleation and crystal growth of the host and inclusion phases. These feedbacks influence the formation of inclusion microstructures in garnet during magmatic crystallization.