Jaka
starting to spend too much on rocks
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Post by Jaka on Feb 16, 2013 21:03:19 GMT -5
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Post by gingerkid on Feb 17, 2013 6:28:01 GMT -5
 Very nice blog with lots of great info, Jaka! I love opals!! Opals are beautiful stones, and learning about them seems endless. Just heard about the Mexican Leopard Opal this week. 
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Jaka
starting to spend too much on rocks
Member since September 2012
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Post by Jaka on Feb 17, 2013 17:46:55 GMT -5
It is definitely among my favorite stones Gingerbrat!
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metalsmith
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Post by metalsmith on Feb 18, 2013 15:20:20 GMT -5
mmm opals are N-I-I-I-I-C-E! They are one of the few gems (arguable) where beauty over-rules all common sense. Unlike other rocks that have intrinsic rarity and beauty dependent on their structure, we all know that at its heart, opal is 'just' quartz. Of course diamond is 'just' carbon and they have in common that their structure is what's important. However diamond structure is regular whereas opal structure isn't and it is the irregularity that shapes and colours their 'play'.
Your blog is well written, but lacks a simple spell-check. It may be better to either - write the blog in word / similar that can be spell checked or (by all means) write it into the blog directly but don't forget to spell-check it / export it for spell checking. You've missed a couple.
Also, I believe that whilst water is fundamental to the formation of opals, it does not all evaporate. Water is retained in between the spherules of opal-quartz and is responsible for the colour and play. The opal quartz spherules size, shape, sorting and orientation determines the water content. However it is known that opals need to be stored properly or they will dry out. This happens over timescales of human lifespans in which case the owner of an opal may discover (probably after many years of storage) that the opal heirloom is now ... err... just quartz!
Hope this helps.
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Jaka
starting to spend too much on rocks
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Post by Jaka on Feb 18, 2013 18:11:30 GMT -5
Thanks Metalsmith, I generally run the spell check through my posts but obviously forgot to with this one! I have also added in that opal does have a small water content.
I believe the drying out you are referring to is known as 'crazing' - when an opal drys out and then cracks. This is why opal is generally stored submerged in water.
Cheers!
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metalsmith
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Post by metalsmith on Feb 21, 2013 14:07:40 GMT -5
Welcome. Typos are easy done and easy fixed.
I heard on the radio (general facts and figures trivia) that opal is around 10% water. Having researched this, I find figures from 1-20% and 3-21% but normally 6-10%.
Apparently the perception that opal 'likes' to be worn is that moisture from the skin maintains it. Yes the crazing comes from drying as the water is unevenly distributed. Opal is intrinsically 'hard', but structurally easy to violate.
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cherdarock
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Post by cherdarock on Feb 23, 2013 19:12:03 GMT -5
OPAL
Chemical Formula:
SiO2•n(H2O)
Composition:
Molecular Weight = 87.11 gm
Silicon 32.24 % Si 68.98 % SiO2
Hydrogen 3.47 % H 31.02 % H2O
Oxygen 64.29 % O
100.00 % 100.00 % = TOTAL OXIDE
Empirical Formula:
SiO2•1.5(H2O)
Environment:
Sedimentary and secondary from the alteration of high silica igneous extrusive rocks.
IMA Status:
Valid Species (Pre-IMA) Prehistoric
Locality:
World wide occurrences. Link to MinDat.org Location Data.
Name Origin:
From the Old Indian upala - "precious stone."
Fire Opal - red
Hyalite - glassy
Hydrophane - porous
Prase - green
Precious opal - opalescent
**Wierd, they don't list Opal as a type of quartz, and the common rule applied to Quartz is that Amethyst is the most precious form...
QUARTZ
Chemical Formula:
SiO2
Composition:
Molecular Weight = 60.08 gm
Silicon 46.74 % Si 100.00 % SiO2
Oxygen 53.26 % O
100.00 % 100.00 % = TOTAL OXIDE
Empirical Formula:
(SiO2)
Environment:
Sedimentary, metamorphic, and igneous rocks.
IMA Status:
Valid Species (Pre-IMA) Prehistoric
Locality:
Very common mineral found world wide. Link to MinDat.org Location Data.
Name Origin:
From the German "quarz", of uncertain origin.
Name Pronunciation:
Quartz
Synonym:
Agate - banded variety of chalcedony
Amethyst - purple
Adventurine - feebly translucent chalcedony
Carnelian - flesh red chalcedony
Cat's Eye - chatoyant
Chalcedony - microcrystalline quartz
Chert - cryptocrystalline quartz
Chrysoprase - apple green chalcedony
Citrine - yellow
Flint - microcrystalline quartz
Hornstone - flint
Jasper - red or brown chalcedony
Moss Agate - variety of chaledony
Plasma - green chalcedony
Prase - leek green chalcedony
Rock Crystal
Rose Quartz - rose colored
Sapphire Quartz - blue colored
Smoky Quartz - brown to black
Tiger Eye - pseudomorph of asbestos
of course they don't list all the quartzes or opals. Fire agate is a big fave too.
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Mattatya
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Post by Mattatya on Mar 5, 2013 2:15:42 GMT -5
Thanks for your blog post on opal. I have never worked with Opal and still a bit confused about them, but your blog has been very helpful.
Cheers,
Mayt
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Jaka
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Posts: 127
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Post by Jaka on Mar 5, 2013 3:30:49 GMT -5
No problem Mattatya, Glad I could help!
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RocknCritter
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Post by RocknCritter on Mar 5, 2013 9:00:45 GMT -5
mmm opals are N-I-I-I-I-C-E! They are one of the few gems (arguable) where beauty over-rules all common sense. Unlike other rocks that have intrinsic rarity and beauty dependent on their structure, we all know that at its heart, opal is 'just' quartz. Of course diamond is 'just' carbon and they have in common that their structure is what's important. However diamond structure is regular whereas opal structure isn't and it is the irregularity that shapes and colours their 'play'. Your blog is well written, but lacks a simple spell-check. It may be better to either - write the blog in word / similar that can be spell checked or (by all means) write it into the blog directly but don't forget to spell-check it / export it for spell checking. You've missed a couple. Also, I believe that whilst water is fundamental to the formation of opals, it does not all evaporate. Water is retained in between the spherules of opal-quartz and is responsible for the colour and play. The opal quartz spherules size, shape, sorting and orientation determines the water content. However it is known that opals need to be stored properly or they will dry out. This happens over timescales of human lifespans in which case the owner of an opal may discover (probably after many years of storage) that the opal heirloom is now ... err... just quartz! Hope this helps. Quartz and opal are polymorphs - that is they have a similar chemistry (Si0 2) yet a different crystal structure. Quartz is trigonal while opal is amorphous. A couple of other common and well known polymorphs include graphite and diamond and andulusite, kyanite and sillimanite. There are literally dozens if not hundreds of minerals that are polymorphs. On the other end of mineralogy are isomorphic minerals - that is minerals that have the same crystal structure yet a completely different chemistry. The garnet and tourmaline groups are the best known examples. And yes, a mineral can be a polymorph to one mineral while being an isomorph to another. Again using quartz as an example of being a polymorph to opal, it is also an isomorph to berlinite (an uncommon mineral). At it's heart opal is "just" silicon dioxide, the same chemistry of quartz, with the molecules arranged in a completelty different manner. This results in one being a mineral with a defined crystal structure (quartz - trigonal) and the other being a mineraloid without a defined crystal structure (opal - amorphous). Quartz doesn't form as spherules and opal doesn't morph into quartz when dehydrated. When opal crazes, it is simply less hydrated opal. Nothing more. Nothing less. The crystal structure didn't change. In order for opal to become quartz, it would require a large change in temperature, pressure and huge amounts of time. Now where's Mel or another trained geologist that can expand upon this?  Mineralogy is the essentially the study of chemistry and crystal structure (geometry) and I'm not particularily good at either of these beyond the basics. |
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metalsmith
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Post by metalsmith on Mar 8, 2013 3:19:57 GMT -5
As a trained geologist, I completely understand your point about the structure. The point I make about opal just becoming quartz is..
The chemical formula of opal is Si02.nH2O, of quartz is Si02
and the difference is .nH2O. Opal is not listed as a polymorph of quartz as it does not share the same chemical formula - they are different.
Dried opal Si02.nH2O becomes Si02 - quartz
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RocknCritter
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Post by RocknCritter on Mar 8, 2013 10:07:17 GMT -5
As a trained geologist, I completely understand your point about the structure. The point I make about opal just becoming quartz is.. The chemical formula of opal is Si02.nH2O, of quartz is Si02 and the difference is .nH2O. Opal is not listed as a polymorph of quartz as it does not share the same chemical formula - they are different. Dried opal Si02.nH2O becomes Si02 - quartz Here is some documentation to clear-up the confusion. From www.quartzpage.de/gen_mod.htmlSiO2 Polymorphs Silica Polymorphs (Network Silicates) Quartz, Low-Quartz, α-Quartz, Alpha-Quartz High Quartz, β-Quartz, Beta-Quartz α-Tridymite, Low-Tridymite
β-Tridymite, High-Tridymite
α-Cristobalite, Low-Cristobalite
β-Cristobalite, High-CristobaliteMoganite (Lutecite, Lutecine) Coesite Keatite Non-Silica Polymorphs Stishovite Seifertite Non-Crystalline Mineraloids
Opal (contains water), with 2 microcrystalline and 2 non-crystalline variantsLechatelierite, Silica Glass From www.mindat.org/min-3004.htmlAlthough it is still (2007) regarded as a valid mineral species for historical reasons, Opal is not a true mineral in the accepted sense of the word as it is either composed of Cristobalite and/or Tridymite or composed of amorphous silica. Opal is broken down into four types: Opal-CT Cristobalite-Tridymite
Opal-C CristobaliteOpal-AG Amorphous -Gel (closely packed amorphous silica spheres form a diffraction grating to create Precious Opal). Opal-AN Amorphous-Network (Hyalite) If de-hydrated opal morphed into quartz, then re-hydrated quartz would become opal. Neither of these scenarios is feasible. If soaking quartz (silica) in water turned it into opal, imagine what all the beaches in the world would be like  A change in the chemistry doesn't alter the mineral structure. Think about pseudomorphs............... |
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Deleted
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Post by Deleted on Mar 8, 2013 13:22:02 GMT -5
Quartz itself usually contains varying amounts of water, a point that seems to conveniently be dropped when the subject turns to the differences between opal and quartz. I'm not sure why the water content is mentioned at all (I agree, the presence of water doesn't contribute to the crystal structure or lack of structure, and so why bring it up at all?).
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RocknCritter
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Post by RocknCritter on Mar 8, 2013 22:05:15 GMT -5
After re-eading the section on temperature on the Silica Polymorph page from quartzpage.de it does state opal may morph to quartz - with sufficient time. I stand corrected.
It does seem odd that while opal is loosing water molecules, the specific gravity increases. Its also odd that while opal would display a break down of the molecular structure through crazing, the hardness actaully increases as quartz.
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gerard
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Post by gerard on Mar 9, 2013 10:41:29 GMT -5
I just cruized through this thread pretty fast, but after reading your blog regarding opal, thought some mention concerning the difference between common and precious opal was warranted.
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metalsmith
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Post by metalsmith on Mar 9, 2013 17:08:34 GMT -5
Quartz itself usually contains varying amounts of water, a point that seems to conveniently be dropped when the subject turns to the differences between opal and quartz. I'm not sure why the water content is mentioned at all (I agree, the presence of water doesn't contribute to the crystal structure or lack of structure, and so why bring it up at all?). Quartz can contain water of crystallisation in varying amounts depending on whether the deposit originates from melt, metamorphism or hydrothermal emplacement. However the amounts of water insitue within the quartz crystal lattice in no way approach the frequent ~10% found in opal. I did include this in an earlier post, but didn't want to confuse the issue any further if it could be avoided. |
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Very nice blog with lots of great info, Jaka! I love opals!! Opals are beautiful stones, and learning about them seems endless. Just heard about the Mexican Leopard Opal this week. 
