Thank you! any tips to recognize secondary biotite? i know crystals are very fine and "spotted" extintion may not occur; but i´ve also observed large biotites aligned as alteration borders in EDM veins, with andalucite, these biotites should be secondary or primary (removilized)?
Sorry for the slow response, I've been in China most of the summer. Sometimes you can see optical differences between primary and secondary biotite (e.g., secondary Fe3+-rich biotite can be greenish and primary Ti-rich biotite can be reddish). But mostly - as you say - I look at texture. Where are the biotite crystals? Are they randomly distributed around the rock and/or concentrated in foliation planes (which is more typical of primary biotite)? Or, do they surround other minerals like garnet, andalusite, etc. (which indicates a later reaction)? Sometimes biotite is intergrown with other minerals, e.g., quartz in a myrmekitic texture that often represents melt crystallization, or biotite can be intergrown with fibrolitic sillimanite as some kind of breakdown product (either prograde or retrograde) after some other mineral. Textures like that are important for interpreting metamorphic reactions and pressure-temperature paths.
Yes, good eye! Those small inclusions with hexagonal outlines are apatite. It's a little hard to see their relief in plane-polarized light, but the interference colors are characteristically low first order gray.
Hello Sir, i have a question, if a biotite in basal section, does it have weak pleochoic? like from dark brown to the darker brown.. And could it be have a low 2V angel so the optical sign is looks like uniaxial? Thanks..
Basal sections do generally have much less pleochroism than sections that cut across cleavages. Sometimes I can't see any pleochroism at all. I have never seen a biotite that looked uniaxial, but I very rarely measure 2V angles.
Thanks for this comment. It's true I don't talk about everything that is visible in these tutorial videos. I like the idea of creating a video that just addresses twinning, though. I'll add it to the list!
Thanks a ton for these slides and explanation
Thank you! any tips to recognize secondary biotite? i know crystals are very fine and "spotted" extintion may not occur; but i´ve also observed large biotites aligned as alteration borders in EDM veins, with andalucite, these biotites should be secondary or primary (removilized)?
Sorry for the slow response, I've been in China most of the summer. Sometimes you can see optical differences between primary and secondary biotite (e.g., secondary Fe3+-rich biotite can be greenish and primary Ti-rich biotite can be reddish). But mostly - as you say - I look at texture. Where are the biotite crystals? Are they randomly distributed around the rock and/or concentrated in foliation planes (which is more typical of primary biotite)? Or, do they surround other minerals like garnet, andalusite, etc. (which indicates a later reaction)? Sometimes biotite is intergrown with other minerals, e.g., quartz in a myrmekitic texture that often represents melt crystallization, or biotite can be intergrown with fibrolitic sillimanite as some kind of breakdown product (either prograde or retrograde) after some other mineral. Textures like that are important for interpreting metamorphic reactions and pressure-temperature paths.
1:57 are those hexagonal intrusions apatite?
Yes, good eye! Those small inclusions with hexagonal outlines are apatite. It's a little hard to see their relief in plane-polarized light, but the interference colors are characteristically low first order gray.
Thank you sir!
Hello Sir, i have a question, if a biotite in basal section, does it have weak pleochoic? like from dark brown to the darker brown.. And could it be have a low 2V angel so the optical sign is looks like uniaxial? Thanks..
Basal sections do generally have much less pleochroism than sections that cut across cleavages. Sometimes I can't see any pleochroism at all. I have never seen a biotite that looked uniaxial, but I very rarely measure 2V angles.
@@mattkohn7819 Thank you so much sir
Hello Sir thanks for these videos. Can u make video showing water and ice under the microscope?
Sorry, I don't have equipment to show ice and water under the microscope. Hexagonal crystal system, though, so it must be uniaxial.
Sir also include twinning when you explain
Thanks for this comment. It's true I don't talk about everything that is visible in these tutorial videos. I like the idea of creating a video that just addresses twinning, though. I'll add it to the list!
Matt Kohn is my religion
This needs to be study