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Single-crystal X-ray study of different vesuvianite samples of known origin shows that different metamorphic grade results in different arrangements of structural rods oriented parallel to the vesuvianite  c  axis, interpreted as .rod polytypism.. There is a systematic dependence of space-group symmetry and rod arrangement on crystallization temperature: P4nc-dominant &lt; 300 °C, P4/n-dominant ~300.500 °C, and P4/nnc &gt; 500 °C. Partial occupancy of the T sites (B, Al, Fe 3+ ) and increased F-content seem to stabilize rod disorder causing P4/nnc space-group symmetry. All studied vesuvianites in calcsilicate rocks and marbles from regional- and contact-metamorphic upper amphibolite facies have disordered rods (P4/nnc symmetry). Electron-microprobe analyses of metamorphic vesuvianites from alpine and non-alpine occurrences, supported by structural investigation, showed that in addition to homo- and heterovalent substitution, partial occupancy of the commonly vacant T sites by B, Al, or Fe 3+ , and the (O 4 H 4 ) 4 . → SiO 4  4-  (hydrogarnet-type) substitutions are significant in nature. With few exceptions, T-site occupancy seems to be restricted to high-grade metamorphic rocks whereas the .hydrovesuvianite. substitution is only found in vesuvianites formed at very low metamorphic grade. The cell parameters of vesuvianite with empty T sites increase with increasing Ti + Mg → 2 Al substitution, and this increase is even more pronounced with increasing .hydrovesuvianite. component. An increase in boron on T sites leads to a decrease of c but an increase in a. Fluorine incorporation and T-site substitution (B, Al, Fe 3+ ) in vesuvianite are coupled with a decrease in hydroxyl groups. This causes vesuvianites to be stable under higher XCO 2  conditions, and in an assemblage with quartz at conditions above the experimentally determined upper stability of quartz + T site vacant, F-free vesuvianite. Optically anomalous vesuvianites have ordered rods and are generally characterized by an intergrowth of P4/n and P4nc domains. In addition to B-rich vesuvianite and wiluite, P4nc-dominant vesuvianites are also commonly optically positive.

Relationship among metamorphic grade, vesuvianite ”rod polytypism”, and vesuvianite composition