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Author Topic:   Quick Questions, Short Answers - No Debate
roxrkool
Member (Idle past 217 days)
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 133 of 650 (510174)
05-28-2009 9:59 AM
Reply to: Message 130 by Blue Jay
05-26-2009 1:39 PM


Re: Birds=Reptiles?
Great post! That was very informative.

This message is a reply to:
 Message 130 by Blue Jay, posted 05-26-2009 1:39 PM Blue Jay has taken no action

  
roxrkool
Member (Idle past 217 days)
Posts: 1497
From: Nevada
Joined: 03-23-2003


(1)
Message 300 of 650 (683587)
12-11-2012 10:03 PM
Reply to: Message 298 by foreveryoung
12-11-2012 2:44 PM


Re: question for geologists
It's been a while since I've looked at thin sections, but what I remember is that quartz is often anhedral, black/white, and clear with no fractures. You can often find small inclusions of other minerals as well as tiny fluid inclusions, and they are often aligned. When deformed, quartz often displays undulatory extinction, sometimes having a granular appearance.

Unless they are interstitial, the feldspars often have twinning and are subhedral to euhedral. And if I remember correctly, cleavage planes are more common and visible, and since feldspars will alter to clay fairly easily, you will often see some sort of 'texture' to the feldspars, particularly along cleavage planes. Fluid inclusions are less common.

Play with the light source. Sometimes you can see cleavages and regular fracture patterns with the feldspars, whereas the quartz will remain smooth.

What I remember about biotite is the brown color (in plane light), the feathery/splintery ends, and bird's eye extinction. Bird's eye extinction is a 'rough' or mottled appearance. I believe this is because the biotite is made up of tiny plates of biotite and when bent up/broken, will go extinct at different angles.

Hornblende, on the other side, often has clean, straight edges (unless being replaced by biotite), is often euhedral/subhedral, and has visible 120/60 degree cleavage, visible as fractures.

Hope that helps some.


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roxrkool
Member (Idle past 217 days)
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 302 of 650 (683741)
12-12-2012 9:43 PM
Reply to: Message 301 by foreveryoung
12-11-2012 11:42 PM


Re: question for geologists
Hmmm... I can't think of a reason why twinning would be more common in euhedral xls than subhedral/anhedral. At least it's not something I ever remember hearing or reading, and I don't think it would be a rule, of sorts. But, hey, it's certainly something to look into.

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