Amphibole discrimination tutorial Optical mineralogy

A series of videos illustrating how to identify different amphiboles. Supported by Boise State University and the US National Science Foundation.
Clinoamphiboles (inclined extinction)
Hornblende: Strongly pleochroic (usually) in browns and greens
Actinolite: Pale green pleochroic
Tremolite: Colorless
Cummingtonite: Colorless, commonly twinned, coexists with hornblende
Orthoamphiboles (parallel extinction)
Gedrite: Gray pleochroic
Anthophyllite: colorless
00:00 Amphibole cleavage (60-120°) in hornblende
00:15 Amphibole cleavage (60-120°) in gedrite
00:31 Amphibole cleavage (60-120°) in glaucophane
00:46 Clinopyroxene with pseudo 60-120° cleavage (oblique section)
01:02 Characteristic cpx end section cleavage with c. 90-90° cleavage
01:17 Intermediate cpx section with c. 80-100° cleavage
01:32 How to find an end section with 60-120° cleavage
02:20 Inclined extinction in hornblende
02:51 Inclined extinction in actinolite
03:22 Inclined extinction and twins in cummingtonite
03:52 Parallel extinction in gedrite
04:22 Parallel extinction in anthophyllite
04:52 Gray pleochroism in gedrite
05:22 Brown pleochroism in hornblende (Ti-rich)
05:53 Green-Bluegreen-Yellowgreen pleochroism in hornblende
06:23 Blue-Lavender-Yellowgreen pleochroism in glaucophane
06:54 Actinolite cores (light green) with hornblende rims (darker green)
07:24 Hornblende core (dark green) with cummingtonite rim (colorless)

[@jorgeandrestaverareyes7381]

Matt, this is golden content

[@giovannimorini2587]

Hello Matt, I was looking for videos on optics and yours have been a great discovery! Thank you!

[@jairprado5538]

Tem uma crivage boa

[@joseangeltorresespinosa7997]

Buen video de mineralogía, pero se podría comentar que tipo roca es ?, un buen saludo desde México.

[@mattkohn7819]

Which type of rock this is depends on which amphibole assemblage we're talking about. Typical glaucophane-rich blueschists and hornblende amphibolites are mostly metamorphosed mafic rocks (basaltic to andesitic in composition). Actinolite is also stable in metamorphosed mafic rocks in the upper greenschist facies before it transitions to hornblende and the amphibolite facies. Fe-Mg amphiboles like cummingtonite (clinoamphibole) and gedrite and anthophyllite (orthoamphiboles) commonly form in hydrothermally altered rocks or their metamorphosed equivalents. Tremolite and actinolite are common in calcic rocks, especially metamorphosed siliceous limestone or dolomite (calcsilicates).

[@joseangeltorresespinosa7997]

@@mattkohn7819 Mil gracias por el aporte, muy detallado, saludos desde México.