Epitaxial nucleation of garnet on biotite in the polymetamorphic metapelites surrounding the Vedrette di Ries intrusion (Italian Eastern Alps)

Preferred orientations of garnet porphyroblasts reveal previously cryptic templating during nucleation

“…5 , illustrating that ( ) gt and ( ) gt are within 16° of ( ) gt and that ( ) gt and (0 1 2) gt are 7° and 18° from (0 1 1) gt , respectively. This generally agrees with previous results 16 – 18 , 21 , suggesting that garnet nucleates epitaxially on the crystal structure of muscovite and/or biotite in the more exotic microstructures presented in those studies. Our results and interpretations further show that epitaxial nucleation is not restricted to unusual microstructures, but instead may be important during crystallization of many or most regional and contact metamorphic rocks.…”

“…S3-7), so inheritance of garnet orientation from muscovite resulted in parallelism with chlorite. These interpretations agree well with previous studies that interpret that garnet can nucleate on the crystal structure of muscovite and/or biotite [16][17][18]21 , with these previous studies showing (1 1 0) gt or ( d-i) Crystal structure models of (0 7 9) gt, (100) ms , ( − 7 7 9) gt, ( − 1 1 6) gt , (1 0 0) ms , and (1 0 0) gt , with dashed lines highlighting relationships between Al in each. The inset shows the 3D orientation of all of the crystal planes.…”

“…These interpretations agree well with previous studies that interpret that garnet can nucleate on the crystal structure of muscovite and/or biotite 16 – 18 , 21 , with these previous studies showing (1 1 0) gt or ( ) gt being within 20° of parallel to (0 0 1) ms,bt . Our results show a clustering in which one of ( ) gt, ( ) gt, (0 1 2) gt , or ( ) gt are preferentially parallel to (0 0 1) ms .…”

“…The interpretations of epitaxial nucleation presented here and in previous studies 10 , 16 , 17 are examples of nucleation via non-classical pathways in geologic materials 2 . Case studies that model nucleation and growth energetics during metamorphism should consider these nucleation pathways and the importance of inherited texture.…”

“…Despite its cubic crystal structure and common form as a porphyroblast, garnet has been previously identified to crystallize via epitaxial and topotaxial relationships with muscovite, biotite, and pyroxene. This manifests as the parallelism of high symmetry crystal planes in one phase to those in another phase 15,16 with the most commonly reported relationship being {1 1 0} garnet // (0 0 1) mica (where '//' = 'parallel' and (hkl) are crystal planes) [16][17][18][19] , though others have also been identified [20][21][22] . However, these examples all either represent unusual microstructures, such as atoll or snowball garnets 21,22 , or formed at extreme environments such as ultra-high pressure metamorphism 17 .…”