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Biotite vs Brown Hornblende - a noob question

 
 
Reply Fri 29 Jul, 2016 07:27 am
Hello everyone!

I have what might be a bit of a stupid question for the seasoned geologists/ petrologists (assuming there are any) on this forum: In a thin section, how do you distinguish biotite from brown hornblende WITHOUT using polarized light, WITHOUT seeing cleavage (I know that an amphibole would have a cleavage of about 120/60) and WITHOUT seeing any idiomorphic grains?

The two seem very similar to me...

P.S.: Apologies if my english sounds kinda odd... I'm not a native speaker. ;P
 
farmerman
 
  3  
Reply Fri 29 Jul, 2016 08:05 am
@AllGoodNamesAreTaken,
substage kighting would show pleiochroism in the biotite. none in amphiboles . Is the biotite/hornblende oriented or in a rock as phenocrysts?
oh yeh, also, how about isotope decay pleiochrosism ( polonium/U/Th on biotite. Hafnium in hornblende (often).
sozobe
 
  2  
Reply Fri 29 Jul, 2016 08:28 am
@farmerman,
Well this might be my favorite exchange in the history of a2k.
farmerman
 
  2  
Reply Fri 29 Jul, 2016 10:02 am
@sozobe,
I was gonna post my internationally famous brat topping recipe for bronzed onions an peppers , but didnt.
(PS the word was sub-stage lighting, not "kighting")

sozobe
 
  2  
Reply Fri 29 Jul, 2016 10:03 am
@farmerman,
Well add that on and I'll remove the "might be" from my previous sentence.
farmerman
 
  2  
Reply Fri 29 Jul, 2016 10:30 am
@sozobe,
do you know how to peel a mangO? I started and I got left with this pile of shredded crap . Mrs F is down at the beach with ome friends and she is in charge of peelingmangoes without losing fruit
sozobe
 
  1  
Reply Fri 29 Jul, 2016 10:45 am
@farmerman,
Hmm, not really. I know what I do, but I don't know if it's correct.

I slice off two big cheeks of mango, then score them so that you have about 1/2" cubes (still attached to the skin). Then separate those from the skin, as close to it as possible.

Then you have to work a bit at the remaining mango left from the center, containing the pit. Just peel it (it's usually also ~1/2") then slice off as much fruit as you can. Ripeness will also dictate how much "good" fruit is available.
InfraBlue
 
  2  
Reply Fri 29 Jul, 2016 12:47 pm
Quote:
Biotite vs Brown Hornblende - a noob question


Whoa.

Your question is definitely not Geology 101.
TomTomBinks
 
  2  
Reply Fri 29 Jul, 2016 10:36 pm
@sozobe,
I just wash it really well and eat it with the skin. gives it some texture.
0 Replies
 
AllGoodNamesAreTaken
 
  1  
Reply Sat 30 Jul, 2016 05:13 pm
@InfraBlue,
Well I was trying to help a fellow student with his homework. He was supposed to describe/ identify a mineral assemblage based on a simple picture of a Dacite in thin section (which I think is a real dick move of the professor). There were some minerals in that picture which looked like either Biotite or Brown Hornblende... so I told him it's probably Brown Hornblende (since Dacites often contain Hornblende). However, I wasn't sure and got kinda bugged by it when my OCD kicked in. So I thought I'd do the only sensible thing and ask some random strangers with unverifiable credentials on the internet... Wink
AllGoodNamesAreTaken
 
  1  
Reply Sat 30 Jul, 2016 05:16 pm
@farmerman,
Quote:
substage kighting would show pleiochroism in the biotite. none in amphiboles . Is the biotite/hornblende oriented or in a rock as phenocrysts?
oh yeh, also, how about isotope decay pleiochrosism ( polonium/U/Th on biotite. Hafnium in hornblende (often).


You lost me at isotope decay pleochroism I'm afraid. Shocked
The thin section in question had a glassy matrix with larger phenocrysts of Plagioclase and Hornblende/Biotite.
farmerman
 
  3  
Reply Sat 30 Jul, 2016 06:25 pm
@AllGoodNamesAreTaken,
you are using a petrographic scope yes?


Radioactive element pleichroism shows up as little "Halos" around the dots of decaying isotopes. As you get further on you will be able to id the isotopes (probably in a monazite xl or a zirc)

The pleichroism of the type pf hornblende is visible in plane polarized light (thats the visible uncrossed polarizer). It usually shows up as brown to green (on extinction)
Can you tell what species plagioclase you hqve?
This should be omething that you learned in the early labs of petrology or optical min.

Good luck.

OH, bytheway go to Smith collwge's website for geosciences

something like www.science.smith.edu/geosciences. Go look around at thwir mineralogy/petrography tables . They show one of the best id charts for most rock forming minerals. ALl the micas and amphiboles (Hornblende being one of the most common) and the plagioclases. It lists the diagnostic stuff in pet -scopes and X ray diffraction.

Good luck-
0 Replies
 
InfraBlue
 
  1  
Reply Sat 30 Jul, 2016 07:20 pm
@AllGoodNamesAreTaken,
My wife took geology this past semester and she didn't have to use polarized light, let alone a petrographic scope, to identify any of the rock samples.
farmerman
 
  2  
Reply Sat 30 Jul, 2016 08:14 pm
@InfraBlue,
Petrographic scopes are a tool that separates the hobbyists from the pros. "Identifying" a mineral isnt the job we count on in the foield. We do confirmations by thin section and radiological analyses. In beginning geo the lab practicals are usually based on obvious rock groups. Telling quartz from pyrite is easy and rock hounds are better than geo students because the geo student may be wondering what is the exact species of quartz or pyrite and why is it there.
What the OP is doing is determining EXACTLY the mineralogical makeup that gives a big clue to the chemical "Thermometry" of how the rock body was emplaced and how deep and ofdten , when. You notice that he already knew that he had biotite and hornblende. He wanted a better clue as to their ID in a thin section not a hand specimen. Optical propeeties are a much more definitive way to determine chemical structure of the minerals thn hand specimens.

Beginning geology for non majors is usually done to fulfill a science requirement were not creating new prospectors. Majors are taught the whys and the details of understanding earth processes, not identifying minerals as an end point., they are taught to apply a whole bag of science tricks (and the theory behind em)
Id muh rather a kid know how to id a mineral with a pet scope than in a hnd specimen. Some minerals form these "PIzza mixtures of defetc crystal lattices so they show us an almost unique xhemical makeup from each location. So a minral like a plagioclase feldspar MUST be identified very precisely according to its pwrecentges of sodium to Calcium Silicates. They form what is known as a "SOLID SOLUTION" which, can hve a whole range of Na /Ca ratios (and each hs a unique name tht only is meaningful when analyzed as a whole rock under a pet scope under polaried light. The thermodynamics of emplacement are tightly tied to those ratios . This gos for hundreds of other minerals (even quartz, which has quite a few unique lattice based polymorphous forms that can only be analyzed by pet scope).
Each one is unique in understanding whether it was emplaced in a pegmatite or melted by a meteorite impct..






ossobucotemp
 
  1  
Reply Sat 30 Jul, 2016 11:27 pm
@farmerman,
I so wish I took it.
0 Replies
 
AllGoodNamesAreTaken
 
  1  
Reply Thu 4 Aug, 2016 08:52 am
@farmerman,
Thanks, I much appreciate your info. Wink
0 Replies
 
AllGoodNamesAreTaken
 
  1  
Reply Thu 4 Aug, 2016 08:55 am
@InfraBlue,
Quote:
My wife took geology this past semester and she didn't have to use polarized light, let alone a petrographic scope, to identify any of the rock samples.


Did she look at thin sections though? Because we also did ore microscopy which is basically reflected light microscopy of polished rock samples. We didn't use polarized light for that either.
0 Replies
 
 

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