Pickersgill, and 2 co-authors, reported in 2015 that shock metamorphism, caused by hypervelocity impact, is a poorly understood process in feldspar due to the complexity of the crystal structure, the relative ease of weathering, and chemical variations, making optical studies of shocked feldspars challenging. Understanding shock metamorphism in feldspars, and plagioclase in particular, is vital for understanding the history of Earth’s Moon, Mars, and many other planetary bodies.
The authors conducted a comprehensive study of shock effects in andesine and labradorite from the Mistastin Lake impact structure, Labrador, Canada. Samples from a range of different settings were studied, from in situ central uplift materials to clasts from various breccias and impact melt rocks. Evidence of shock metamorphism includes undulose extinction, offset twins, kinked twins, alternate twin deformation, and partial to complete transformation to diaplectic plagioclase glass. In some cases, isotropization of alternating twin lamellae was observed. Planar deformation features (PDFs) are notably absent in the plagioclase, even when present in neighboring quartz grains. It is notable that various microlites, twin planes, and compositionally different lamellae could easily be mistaken for PDFs and so care must be taken.
A pseudomorphous zeolite phase (levyne-Ca) was identified as a replacement mineral of diaplectic feldspar glass in some samples, which could, in some instances, also be potentially mistaken for PDFs. The authors suggested that the lack of PDFs in plagioclase could be due to a combination of structural controls relating to the crystal structure of different feldspars and/or the presence of existing planes of weakness in the form of twin and cleavage planes.
The authors concluded that the most widely recognized diagnostic shock metamorphic feature in plagioclase feldspars at Mistastin is diaplectic feldspar glass. Inferences about the degree of shock can be made based on the degree to which single crystals have been isotropized (the entire crystal, part of the crystal, only alternate twins, etc.). Because diaplectic glasses lose more internal structure with increasing shock level (e.g., Lambert and Grieve 1984), it appears that at higher pressures twins become less evident and the degree of chemical homogenization increases—as diaplectic glasses get closer to being monomineralic melt glasses. Other features, such as fracturing and undulose extinction, can be indicative of shock metamorphism but are definitely not diagnostic.
Further investigation into the nature of planar deformation features in plagioclase needs to involve the investigation of twin and cleavage planes by finer scale techniques to identify if glass is present along those planes, indicative of PDFs, as in quartz, parallel to rational crystallographic planes but masked by coincident pre-existing planar features in the mineral.
The authors also encouraged all workers to report the chemical composition of the feldspars studied to investigate the potential role of this parameter in PDF formation.
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