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 Post subject: Garnet With Rutile??
PostPosted: Fri May 29, 2009 7:56 am 
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Hello Everyone,
I was looking at a stone and it turned out that it was showing double refraction when seen under the polariscope, but when i checked the R.I. using polarizing filter the R.I. stayed constant at 1.77 no matter how i turned it....I guessed it was garnet showing ADR...but on checking in microscope the stone was showing rutile silk as seen in rubies...the criss cross hatches...I want to know can garnet have silk like inclusions of rutile...is there any way to confirm it is a garnet...the dichroscope showed no pleochroism and the it was inert to u.v so definately not ruby.. i cannot rely on the s.g. because the size is small and the error gets too large...can anyone suggest a better method of testing..

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PostPosted: Fri May 29, 2009 8:18 am 
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hi,
i can cornfirm the presence of silk in garnets. i saw some rodolites full of it with the same pattern usually visible in rubies.

Image

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PostPosted: Fri May 29, 2009 9:06 am 
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Yep! That's the one Alberto...Distinctly similar...I was wondering how it is oriented in the hexagonal pattern when the garnets are from cubic system..at least i know that my stones are definately garnets!

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PostPosted: Fri May 29, 2009 9:38 am 
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It may look surprising, but the cubic system is actually 'defined' by the fact that it has at least three axis with three-fold symmetry. (I think a fourth will always exist once you have found 3). If you spin a rhombic dodecahedron (a classic shape for garnet crystals) around, you can 'look down' on one of the points where 3 vertices meet, and you will see a hexagonal outline. Look 'down' at a point where 4 vertices meet, and you will see a square outline.

As an analogy, you can look down 'diagonally' at a cube from one of the corners down to the exact opposite corner. The outline will be hexagonal as well, with the 3 vertices joining the center point to 3 of the 6 ouline corners.

Try it out here.

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PostPosted: Fri May 29, 2009 10:59 am 
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Hmm.... you do make sense...I guess that's the reason euhedral crystals of garnet in thin section look hexagonal in shape...

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PostPosted: Fri May 29, 2009 1:40 pm 
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In garnets, rutile needles will be found parallel to the four directions of the rhombododecahedron. They are quite common.


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PostPosted: Fri May 29, 2009 2:13 pm 
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Also Rutile in garnet tends to intersect at angles of 110 degrees and 70 degrees rather than the 60 degrees normal to hexagonal crystals

Pretty easy to measure with a protractor if you can post a pic on a computer screen

Hey Keirkof sounds like your ready for that exam...are you sitting in June?

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PostPosted: Fri May 29, 2009 2:35 pm 
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Frank wrote:
Pretty easy to measure with a protractor if you can post a pic on a computer screen


maybe you can try with the pic i posted above. :D
ciao Frank
Albé

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PostPosted: Fri May 29, 2009 8:06 pm 
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Sometimes garnets with rutile inclusions show bright reflective characteristics like this in darkfield illumination.

Image

I've never seen this mentioned in ruby, but it may exist. Your observations are screaming garnet. There are some other simple tests to confirm, but I don't think it's necessary at this point.


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PostPosted: Sat May 30, 2009 1:27 am 
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Frank wrote:
Hey Keirkof sounds like your ready for that exam...are you sitting in June?


I hope you're right :) I made arrangements with Gem-A and Peter to sit in June in Arnhem. When I get back from three weeks off this Monday I should find my exam confirmation form in the mail pile. Then a fortnight later it'll be bizness time. :)

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PostPosted: Sat May 30, 2009 8:54 am 
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Yup the stone is garnet indeed and are showing A.D.R....i did check the angle and it turned out as mentioned...I even checked in U.V. Thanks for the info guys!!

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