John Faithfull๐ŸŒ๐Ÿ‡ช๐Ÿ‡บ๐Ÿด๓ ง๓ ข๓ ณ๓ ฃ๓ ด๓ ฟ๐Ÿงก โœŠ๐ŸผโœŠ๐Ÿฟ
#DogWalkingGeology from this morning. Here's a lovely wee layer of pale blue elongate kyanite crystals, pale pink elongate andalusite, plus some wee brown round garnets, in schist from Ardalanish Bay. Although the andalusite and kyanite look happy together, they are not actually in equilibrium: this layer has "frozen" during the slow process of converting kyanite to andalusite. Reactions are all about the kinetics! #Metastable #Mull #Moine #RossOfMullGranite #Geology #Minerals #Al2SiO5 #Kinetics
Jul 25, 2023 at 12:11pmWeb
Show threadVicky VeritasJul 25, 2023
@FaithfullJohn Fascinating, John! Thank you!
Show threadJohn Faithfull๐ŸŒ๐Ÿ‡ช๐Ÿ‡บ๐Ÿด๓ ง๓ ข๓ ณ๓ ฃ๓ ด๓ ฟ๐Ÿงก โœŠ๐ŸผโœŠ๐ŸฟJul 25, 2023
@vickyveritas This locality is just sooooo good! You can see in the pic that many individual crystals are part-kyanite and part-andalusite, which is just ๐Ÿคฏ ๐Ÿคฉ ๐Ÿฅฐ More on this in John Wheeler's paper https://academic.oup.com/petrology/article/45/4/835/1529671
Disequilibrium in the Ross of Mull Contact Metamorphic Aureole, Scotland: a Consequence of Polymetamorphism

Abstract. The Ross of Mull pluton consists of granites and granodiorites and intrudes sediments previously metamorphosed at amphibolite facies. The high grade a

OUP Academic
Show threadVicky VeritasJul 25, 2023
@FaithfullJohn This is awsome! Thank you for the link to the paper!
Show threadcobalt going beige todayJul 25, 2023
@FaithfullJohn @vickyveritas seems to be weathering fast!
Show threadBob DownieJul 25, 2023
@FaithfullJohn My undergraduate field trip to Glen Clova is coming back to me. Barrovian?
https://www.oxfordreference.com/display/10.1093/oi/authority.20110803095448739;jsessionid=94DD686DAD1B9CDADBBE9200EB03E4F7
Show threadJohn Faithfull๐ŸŒ๐Ÿ‡ช๐Ÿ‡บ๐Ÿด๓ ง๓ ข๓ ณ๓ ฃ๓ ด๓ ฟ๐Ÿงก โœŠ๐ŸผโœŠ๐ŸฟJul 25, 2023
@geomannie Yes and no. Kind of a tiny bit, in that "Barrovian" refers to the kind of regional metamorphic minerals that grow in "normal" pressure-temperature profile thickened crust (as in the eastern Dalradian rocks in Glen Clova). The kyanite here grew under that kind of metamorphism. But in the "Moine" package of rocks, not the "Dalradian" package of rocks, and probably a bit later, and at higher pressure than the Clova Dalradian. .../more
Show threadJohn Faithfull๐ŸŒ๐Ÿ‡ช๐Ÿ‡บ๐Ÿด๓ ง๓ ข๓ ณ๓ ฃ๓ ด๓ ฟ๐Ÿงก โœŠ๐ŸผโœŠ๐ŸฟJul 25, 2023
@geomannie .../cont'd... But the later andalusite overprint here in Mull is due to the thermal aureole of the Ross of Mull Granite pluton. This is quite restricted in extent, and not "Barrovian". Although lots of metamorphism is neither purely thermal from magma, or regional without magma, so the "Barrovian"/"non-Barrovian" distinction can be a bit blurred sometimes. ๐Ÿค“ I'll stop now ๐Ÿ˜
Show threadBob DownieJul 25, 2023
@FaithfullJohn As a softy these ideas are scouring the dregs of my student days but thanks for the explanation. Andalusite did seem a bit off beat for Barrovian metamorphism so thanks for the explanation.