|
|
Joined: Aug 2000
Posts: 3,409
Carpal Tunnel
|
Carpal Tunnel
Joined: Aug 2000
Posts: 3,409 |
|
|
|
|
|
Joined: Nov 2000
Posts: 866
old hand
|
|
old hand
Joined: Nov 2000
Posts: 866 |
[smug -e] I have a fulgurite - found it in a sand dune during a First Year geology excursion at Long Reef, one of Sydney's northern beaches.
It isn't glassy, the exterior being quite granular. The grains lining the tube seem welded together.
I originally thought that it may be a fossil crab burrow, but numerous people have since concurred that it's a fulgurite.
stales
|
|
|
|
|
Joined: Nov 2000
Posts: 866
old hand
|
|
old hand
Joined: Nov 2000
Posts: 866 |
Tektites are another form of naturally occuring glass. The consensus these days is that they are cooled droplets of the earth's crust flung up during an asteroid impact - or during large volcanic events. Thailand is well known for dark green / black tektites; in Australia we have black (smoky when polished) ones (called Australites - got a nice one of these from Kalgoorlie); but the prettiest of all are Moldovites - champagne bottle green and often intricately etched. Just to throw in a word link - there's a whole terminology associated with tektites. Things such as buttons, flanged buttons, teardrops, partial flanges, dumbells - let alone the various types of tektites. Have a squiz at the URL given... Good URL at http://www.crystal-world.com/html%20pages/meteorite%20pages/tektites/glassy_tektite.htm It mentions several other types as well. BTW - all glass is, in geological terms, quite unstable. Glassy rocks and tektites relatively rapidly break down to clay. stales
|
|
|
|
|
Joined: Apr 2000
Posts: 3,065
Carpal Tunnel
|
|
Carpal Tunnel
Joined: Apr 2000
Posts: 3,065 |
In reply to:
i think obsidian is formed by volcanos-- certainly, it is almost always black.
Obsidian is black? One of my favourite exhibits at the Museum of Mankind in London (I have this vague idea the museum is not there any more, having been reabsorbed into the British Museum) was a "glass" Aztec skull. I'm sure it was described as being made of obsidian but was the normal transparent non-colour of ordinary glass. It really was beautifully carved and a fascinting object.
Bingley
Bingley
|
|
|
|
|
Joined: Jun 2001
Posts: 2,636
Carpal Tunnel
|
|
Carpal Tunnel
Joined: Jun 2001
Posts: 2,636 |
Ackshully® cheque is in the mail, Mav, now that I see my options Thanks, stales!, the pendant in question is really Moldovite. It is the color and wiggliness (champagne green and intricately etched) that you described. I couldn't remember what he called it, but I knew it was not of this world. 
|
|
|
|
|
Joined: Sep 2001
Posts: 6,296
Carpal Tunnel
|
|
OP
Carpal Tunnel
Joined: Sep 2001
Posts: 6,296 |
Ah, Stales, you should be smug! To have a creation of lightning in your hand! That's a great gift of nature, isn't it?
I just wish there were better names for fulgurite. Would love to know more names--even in other languages--for this lightning glass.
Best regards,
WW
|
|
|
|
|
Joined: Jan 2001
Posts: 13,858
Carpal Tunnel
|
|
Carpal Tunnel
Joined: Jan 2001
Posts: 13,858 |
I have read that glass is a super-cooled solution. When it is molten, the rate of cooling makes big differences in its properties. It has to be cooled very slowly.The huge panes used in modern buildings have to be tempered, or they would shatter too readily. The glass in automobile windshields is treated so that when it shatters, none of the pieces are bigger than a fingernail. The old windshields could make swords that could inflict gruesome wounds. When I was a boy testing urine specimens, the test-tubes were exasperatingly fragile. They had to be warmed up very slowly in the alcohol burner flame, or they would shatter and spill the urine. So, the invention of Pyrex glass was a very great blessing.Adding some borax made a very great improvement in both thermal and mechanical shock resistance.
And the first telescope lenses were very disappointing because of rim of color, called chromatic aberration. Many years went by before it was discovered that special optical glasses could greatly improve the quality of the image. That's why your camera lens is not just a single piece of glass, but three of four lenses of different composition nested together to avoid both spherical and chromatic aberration.
P.S. The big telescope mirrors take as much as six months to cool, to minimized internal stresses.
|
|
|
|
|
Joined: Nov 2000
Posts: 866
old hand
|
|
old hand
Joined: Nov 2000
Posts: 866 |
Thanks for the info wwh - concurs with what I've heard. Taking it further though, somebody once told me that the fragments of safety glass (basketball's "ice shower") had achieved solid status (from being a super cooled liquid).
Also heard that glass retains its fluidity throughout its "working" life. One of the characteristics of a fluid is the ability to flow - which is exactly what glass does. Apparently the glass in windows ever so slowly succumbs to gravity, to the point that the window panes in old churches (frinstance) are thicker at their base than at their top.
Just another couple of bits of trivia about the colouring of glass....
A friend of mine collects old (say 1st half of the 20th century) glassware. Things like cake plates and so on. He leaves them on top of his garage roof in the blazing Perth sun for a couple of years until they turn that beautiful amethyst colour I presume we've all seen. They look stunning. Obviously (help Bean!) it's the sun's gamma(?) radiation that sets off the trace elements in the glass. My friend cites it as being due to the presence of uranium in the glass - but obviously not to the level that produces the deep yellows mentioned earlier in this thread. He also told me that the inclusion of uranium in glass was banned in the 50's (unsure of the date).
Gamma radiation is used commercially to produce coloured quartz - presumably in a similar (but shorter) process than my friend's garage roof! Clear quartz is turned a whole range of yellows, lilac and so on through this process. Some of you may have seen the bi-coloured gem "ametrine" - half lilac, half yellow. Very pretty but, for me, being artificial, not that desirable.
In a similar fashion, the vast quantities of amethyst produced by Brazil and India are often heat treated (I believe they are simply baked in an oven) to produce butterscotch orange "citrine" or black "morion". Both are naturally occuring varieties of Quartz but, IMHO, buyers should be aware that they are acquiring an "enhanced" or "altered" product. (It'll never happen so I won't lose any sleep over it!!)
Finally, whilst I haven't seen (first hand) the reds and yellows discussed above, the mineral world has a couple of magnificent offerings - realgar and orpiment. These are closely related sulfides of arsenic with stunning depth of colour. They were used historically by icon artists, the crushed mineral being mixed with egg yolk to make the paint.
Well that turned out to be a bit of a mixed bag hey!
stales
|
|
|
|
|
Joined: Dec 2000
Posts: 1,055
old hand
|
|
old hand
Joined: Dec 2000
Posts: 1,055 |
Wow, what an astounding array of information on glass. I knew that glass was a very viscous liquid (at least most glass) and always wondered about how much activity really occurs in a pane over the years, so thanks for that info stales.
I don't really know too much about physical glass, but I was so intrigued, that I decided to look it up for yet further clarification:
'Any of a large class of materials with highly variable mechanical and optical properties that solidify from the molten state without crystallization, are typically made by silicates fusing with boric oxide, aluminum oxide, or phosphorus pentoxide, are generally hard, brittle, and transparent or translucent, and are considered to be supercooled liquids rather than true solids.' AHD
Aside:
Glass is also the name of a famous American minimalist, (Philip), who wrote a piece for 12 keyboards, and during the first performance thereof a lady started screaming and there was an awful ruckus the likes of which hadn't been seen since 'The Rite of Spring' debut.
|
|
|
|
|
Joined: Nov 2000
Posts: 866
old hand
|
|
old hand
Joined: Nov 2000
Posts: 866 |
Fulgurite pix - NOTE: Commercial site (not stales related but) http://tektitesource.com/Fulgurites.htmlFTR, mine's like the top ones. stales
|
|
|
|
Forums16
Topics13,913
Posts229,330
Members9,182
| |
Most Online3,341
Dec 9th, 2011
|
|
|
0 members (),
1,577
guests, and
1
robot. |
|
Key:
Admin,
Global Mod,
Mod
|
|
|
|
