|
Post by sheltie on Oct 10, 2013 15:18:00 GMT -5
I can't figure out what this is. It looks like snowflake but not obsidian or rhyolite. Any other ideas?
|
|
|
Post by deb193redux on Oct 10, 2013 15:19:21 GMT -5
firecracker obsidian
|
|
|
Post by sheltie on Oct 10, 2013 15:31:21 GMT -5
I would normally yield to your ID - and I understand that seeing a picture isn't NEARLY as good as having a slab in your hands - but I have to disagree that this is obsidian. UNLESS, the look and texture of all obsidian isn't the same. I originally thought this was just granite and wasn't going to slab it, but I'm glad I did.
|
|
|
Post by deb193redux on Oct 10, 2013 21:58:02 GMT -5
|
|
|
Post by sheltie on Oct 11, 2013 8:54:48 GMT -5
Right now the rough is still covered with pudding. When I clean it up I'll post a picture of it (if I can remember!). The slab sure looks like fireworks obsidian, so I guess we'll know better when I post a picture of the rough. Believe me, it looks NOTHING like the obsidian with which I'm familiar.
|
|
|
Post by deb193redux on Oct 11, 2013 10:24:19 GMT -5
Looking forward to the picture. You can also compare rough for both fireworks and similar starry-night obsidians at greatRough.com www.greatrough.com/rough_obsidian_fireworks.shtmlwww.greatrough.com/rough_obsidian_starry_night.shtmlUsually there is some dark areas (for both these types) along natural edges that show the shiny glass pattern. Starry Night has smaller less-bursty dots and a web/crackle pattern in the dark part. There is also more evidence of striation, likely flow lines in the obsidian. Fireworks, at its best has clean translucent dark areas with well separated well-articulated bursts. There is less evidence of bursts in layers. I think flow patterns are still there, but the material flowed slower and cooled more slowly. The bursts had more time to develop large crystals.
|
|
|
Post by pghram on Oct 11, 2013 10:37:28 GMT -5
I does look like fireworks ob. but what ever it is, it sure is nice.
Rich
|
|
|
Post by tntmom on Oct 11, 2013 11:40:45 GMT -5
The star-bursts in Fireworks tend to be much softer than the typical snowflake obsidian types. It will texture quite a bit if you don't use a super light touch. When cabbed properly though it is an absolutely beautiful opaque obsidian. And.... your slab picture looks identical to the Fireworks I have
|
|
|
Post by sheltie on Oct 11, 2013 15:34:53 GMT -5
Looking at the pix that Daniel has above, I agree that mine is Fireworks obsidian. My rough looks very much like those. You CAN teach an old dog new tricks after all!
|
|
|
Post by 1dave on Nov 12, 2013 15:19:46 GMT -5
Obsidian, as all rocks, forms under a variety of conditions with varying contaminating materials. To form in the first place, the magma must be very high (75+%) in SiO2.
Under high pressure very hot water remains water. 2-4% water allows SiO2 to crystallize as cristobalite spheres.
If very low in alkali minerals, feldspars don't form, and the spheroids become "snowflakes." More alkali's, spheroids continue to grow around smaller spheroids and can become thundereggs.
If the water content gets as high as 7%, perlite is formed instead of obsidian. "Apache Tears" are places that were low in water as the perlite formed.
"Pitchstone" is an intermediate form that the Spencer Opal Thundereggs formed in.
Cristobalite is softer than obsidian, so tends to undercut.
|
|
|
Post by deb193redux on Nov 12, 2013 20:07:12 GMT -5
I am not sure the water is in the flow at the time it flows. I think it starts as obsidian, and the hydration occurs from ground water. In this view, the Apache tear is an area where the groundwater did not penetrate. Or this is what I understood from reading.
|
|