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 Post subject: what causes the closed c-axis?
PostPosted: Sat Jan 27, 2007 4:29 am 
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hi,when you do the white paper test for examining rough for colour in all directions,what causes a closed c-axis in dichroic material?
is it due to just a clarity/inclusion issue in that direction,or more to do with where the molucules are arranged differently along the c-axis?eg more compressed in the one direction.
does it occur more with rough that is highly dichroic?

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PostPosted: Sun Jan 28, 2007 1:04 pm 
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Hi,

It is due to strong selective absorption in the direction of the c-axis. In other words, the direction that would show one of the dichroic colours (the ordinary ray) is completely (or almost completely) absorped and will thus appear black (or very dark).
You will see this in Tourmaline (none others that I know of).


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PostPosted: Mon Jan 29, 2007 6:05 am 
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Great question dingtong, it's something that I've wondered off and on for a while and never thought to ask. Thanks for the reply Doos, now I know!

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PostPosted: Wed Jan 31, 2007 4:34 pm 
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Hi,

Can't imagine why I forgot, but the mineral herapathite also exhibits this. It was that mineral that was used in the first polaroid filters as one ray was completely absorbed. The remaing ray comes out polarized (instead of recombining to unpolarized light).


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PostPosted: Wed Jan 31, 2007 6:04 pm 
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Doos, is there anything left that you DON'T know about gemology/mineralogy? ;)


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PostPosted: Wed Jan 31, 2007 6:08 pm 
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Heh, plenty. I wish I could remember what I forgot.


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PostPosted: Wed Jan 31, 2007 6:14 pm 
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Doos wrote:
Hi,

Can't imagine why I forgot, but the mineral herapathite also exhibits this. It was that mineral that was used in the first polaroid filters as one ray was completely absorbed. The remaing ray comes out polarized (instead of recombining to unpolarized light).


Wasn't that discovered by some mineralogist named Nichols? Thus the term "crossed Nichols."


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PostPosted: Wed Jan 31, 2007 6:21 pm 
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Hi,

You are correct, that also works as a polarizer indeed, but it's based on clever use of critical angles. The herapathite filters created by Land are polarizing sheets (should have mentioned "sheets").

p.s.: I believe "polaroid" filters is a trademark.


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PostPosted: Thu Feb 01, 2007 8:15 am 
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thanks doos,
forgive my ignorance,so you only have to worry about a closed c-axis on the 2 minerals you mentioned?it doesnt occur in corundum?
regarding rough corundum:will it generally be lighter or darker along a certain axis?
(disregarding inclusions,shape etc)


many thanks

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PostPosted: Thu Feb 01, 2007 8:49 am 
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dingtong wrote:
thanks doos,
forgive my ignorance,so you only have to worry about a closed c-axis on the 2 minerals you mentioned?it doesnt occur in corundum?
regarding rough corundum:will it generally be lighter or darker along a certain axis?
(disregarding inclusions,shape etc)


many thanks


Since corundum is pleochroic (dichroic). There are two rays in the corundum stones that has a affect on the colour, they absorb light differently and when you turn the stone you could see a slight change of the shade of colour. That is dchroism, and you could expect to see this in corundums (exept yellor or colourless ones).

In ruby the prefred colour (and in sapphire) is the colour seen in the ordinary ray, so to gain maximum value of a ruby/sapphire you should cut the stone with the table perpendicular to the c-axis.


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PostPosted: Wed Feb 21, 2007 11:59 pm 
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Are there achroites with a closed c? I haven't seen one personally.


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