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Author Topic: Tectites in Jamaica?  (Read 24654 times)

Offline RD

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« Reply #25 on: 11/09/2008 03:18:15 »
On the left are bits of casts made from the interior of ammonite type shells, (i.e. shell is mould).



your similar find is on the right

[Your smooth "whorl" finds are consistent with being casts of the entire interior of helical shells]
« Last Edit: 11/09/2008 03:46:31 by RD »
 

Offline JimBob

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« Reply #26 on: 11/09/2008 04:54:42 »
The pink rock with the quarts looking vein running down the middle of it is a textbook example of contact metamorphism. The isolate pink piece in the middle with the white rind around it is typical of this intrusive type of contact metamorphism. See Bass's post for another example. Both of these are textbook examples of contact metamorphism - don't be thrown off by rounded nature of this particular specimen.

Bass's Post - http://www.thenakedscientists.com/forum/index.php?topic=16816.msg193185#msg193185
 

Offline susanshirleyjamaica

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« Reply #27 on: 11/09/2008 15:04:16 »
Hi JimBob,

I really am beginning to understand why there are so many beautiful and diverse rocks on our beach.  You said... don't be put off by the rounded nature of my rock.  How might it (and almost every other rock on the beach) become rounded?  Is it river abrasion?  Or wave action rolling them in and out of the sea?  Did they break off a rock face somewhere in the mountains?  Our river, the Rio Nuevo, has a wide flood basin and often goes into spate... renewing beachcoming opportunities about every two months or so.

Below is a picture of cliff face and beach...

 

Offline susanshirleyjamaica

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« Reply #28 on: 11/09/2008 15:07:44 »
This is a picture of the river mouth.  It changes often.



 

Offline JimBob

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« Reply #29 on: 12/09/2008 02:24:52 »
"By Jove, I think she's got it!"

      -------  Pygmalion
 

Offline susanshirleyjamaica

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« Reply #30 on: 12/09/2008 13:20:23 »
Ya... I really am getting it... at least enough so that when I walk along the beach I can identify (in a very broad way) many of the rocks that had me completely perplexed not so long ago.  Most helpful has been gaining a better understanding of how metamorphism works, and the very many different ways that it does work.

Thanks a whole lot... all you guys!
 

Offline Bass

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« Reply #31 on: 12/09/2008 20:09:02 »
Susan, I like this breccia




Note that the clasts are not rounded, and how the layers, even though fragmented, are still continuous (black fragments in oval).  Also very little in the way of reaction rims around the fragments.

My interpretation- this rock was fractured (broken apart) and fluids deposited quartz in the open spaces.  Probably happened numerous times since the breccia appears to be matrix supported.  While breaking apart, the two sides did not otherwise move relative to each other- if they had, the dark layer of fragments would have been disrupted.
 

Offline susanshirleyjamaica

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« Reply #32 on: 13/09/2008 02:28:59 »
I also like that Breccia... I'm fascinated with quart matrix!

Actually, my favourite rock, at this time anyway, is the large... breccia (?) with crystalline matrix.  It actually has in it broken pieces of what looks to me like volcanic rock, almost black and bubble pitted.  There are some lighter volcanic pieces as well. They are obviously broken.

Am I correct in assuming the left hand piece is volcanic?  It is palm size and quite light.

The dark smaller rock I think is basalt?  The piece on the far right appears to be several large, amber coloured and fairly transparent crystals jammed up together. The detail doesn't really show its transparent nature.



 

Offline Bass

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« Reply #33 on: 13/09/2008 04:50:57 »
large rock may be flow breccia (chunks break off as the lava flows) that was cemented by later minerals.  Hard to tell from pictures, but if the lower left rock is light and full of holes, as you describe, probably volcanic as well.  Center looks like black rock- could be basalt.  Can you see any crystals in it?

Las tork looks like some sort of jasper or chert.  Is it harder than a knife?
 

Offline Bass

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« Reply #34 on: 13/09/2008 04:56:12 »
Yes, that is it!  Thanks.  Now... is it unusual for such a band of ophialite to be exposed?  Am I especially blessed to have such a variety on one rock beach? 
So here are three more I picked up yesterday evening (my husband is beginning to think I am nuts!). The one on the left appears to have volcanic rock embedded in a sort of quartz.  Next is just another breccia in paradise.  On the right is an amazing rock with pink speckles and exposed quartz crystals.



So now I am going to bring out my fossils.  :-)

Bottom breccia is definitely clast supported, perhaps jasper?

Love the cockscomb quartz in the "pink" rock.  Obviously open spaces between the fragments allowed large quartz crystals to grow radially around each one- then a thinner band of darker (green?) minerals.  Are the clasts corroded at all (replaced by the quartz)?  Or do the clast boundaries well defined?
 

Offline susanshirleyjamaica

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« Reply #35 on: 13/09/2008 16:12:11 »
Hi Bass,

I didn't think about the "pink rock" as having clasts... but I guess on close look, maybe it does.  If so, the clasts are not distinct and their edges not well defined.  The dark lines around the quartz crystals is black and has some very small jet black crystals, which I didn't notice before.  Maybe these "other view" pictures might help.

 

Offline susanshirleyjamaica

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« Reply #36 on: 13/09/2008 16:14:01 »
View of the other side

 

Offline susanshirleyjamaica

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« Reply #37 on: 13/09/2008 16:37:26 »
The dark rock that I thought might be basalt has no visible crystals in it... in fact is has very finely scattered pitting.

In the "flow breccia" the clasts are well defined. There may be other small rock types in it, but the darker ones, as well as the larger lighter ones, seem most certainly volcanic, as far as I can tell anyway.  Here is a close up.

 

Offline susanshirleyjamaica

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« Reply #38 on: 13/09/2008 16:53:46 »
One more... the rock that I thought was crystals and you asked if it was jasper or chert-like.  Maybe chert, but looks different.  Here's two more details.  The second detail show it with the other rock in my collection which it most resembles.. But the texture is different... maybe it is chert, if chert can be transparent?  A knife will not scratch it unless I press real hard.  I'm not sure I am doing the test right.





 

Offline Bass

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« Reply #39 on: 13/09/2008 21:15:59 »
Looks like chalcedony or opal.  With all this quartz/silica, and mineral deposits nearby?
 

Offline susanshirleyjamaica

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« Reply #40 on: 14/09/2008 00:24:41 »
shucks... the window doesn't want to accept my picture... even though it is only 83kb!
I'll try again later.
 

Offline Bass

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« Reply #41 on: 14/09/2008 00:54:41 »
My guess is quartz-chalcedony-opal; conchoidal fracture is a sure sign.  Calcite scratches easily with a knife, quartz does not.
 

Offline susanshirleyjamaica

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« Reply #42 on: 14/09/2008 01:47:59 »
Cool.... That sounds good to me.  But is it the same sort of opal that they make semi-precious jewelry stones from?
 

Offline Bass

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« Reply #43 on: 14/09/2008 04:35:40 »
Chalcedony, agate, chert (also flint) and jasper are all varieties of cryptocrystalline (extremely fine-grained) quartz.  Even though these are basically the same, I think of agate as banded, jasper as colored by impurities, and chert as forming by diagenetic processes.  Opal is amourphous silica (a bit of water messes up the structure).  There is opal, and there is precious opal- minor structural differences make the difference.
« Last Edit: 14/09/2008 04:39:43 by Bass »
 

Offline susanshirleyjamaica

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« Reply #44 on: 14/09/2008 14:46:45 »
I'm guessing these are chalcedony... can I get away with calling them Jamaican Opal?

 

Offline susanshirleyjamaica

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« Reply #45 on: 14/09/2008 14:54:25 »
On to a completely different rock... The large one looks like a blobby conglomerate or nodule seems to be cemented with limestone.  Forgot to put scale, it is about 8" across.  The two small ones are different sides of the same palm rock that looks like two pieces of plasticine mushed together.




« Last Edit: 14/09/2008 15:00:01 by susanshirleyjamaica »
 

Offline JimBob

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« Reply #46 on: 15/09/2008 01:06:10 »
Is the blue color partly due to the light? Or is the rock actually blue in color. It looks like a clastic mud-crack rip-up then redeposited rock but that doesn't fit with the rest of the beach.
 

Offline susanshirleyjamaica

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« Reply #47 on: 15/09/2008 02:01:35 »
I thought it was pretty strange as well... I've never before seen anything like it.  The clasts are dark gray with a suggestion of blue to blue-green. The matrix is yellowish limestone (the larger areas of matrix do not fizz, but the very fine cracks do) that sure looks like it was once mud!  Tell me how these are usually formed and maybe it will give me a clue. 

It has a cast hole that looks as if it might have held a fossil.

Blob-Rock 1

 

Offline susanshirleyjamaica

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« Reply #48 on: 15/09/2008 02:20:08 »
Blob-Rock 2

 

Offline susanshirleyjamaica

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« Reply #49 on: 15/09/2008 02:23:42 »
Blob-Rock 3

 

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