ETHIOPIAN OPAL formation - OBSIDIAN ?

[OpalLoverCH]

Hello everybody

Does someone know something about the formation/genesis of Ethiopian opals ?
I mean the ones which have brown body color (and sometimes incredible play of color). The name is also opalised `thunder eggs`, I think.
I`m nearly sure that this kind isn`t sedimented and I think it looks like obsidian (volcanic glass) or something like that. I have heard that obsidian could contain to 70% silica or more. But how does the silica spheres form and assemble in the (viscous) obsidian (glass), if it is obsidian ?
Or is it originally suspended in water and does the water evaporate later ?
Could one say that the opalished thundereggs are more like `opal glass` than real opal ?
Could it be that it is a lattice of ordered crystals (cubic, hexagonal, spheres or something else) but not amorphous silica spheres ? Are thunder eggs `bullets` of the volcano or bubbles ? Ejected from the volcano ? Or injected into volcanic ash ? Who does know ?
Who can help ? I am very interested in opals and there is not much to read about this special variety. Please give me input, it would be great...
Thank you so much and by,

OpalLover

 

[Richard M.]

Hi,
You''ve asked a lot of complex questions -- too many to answer briefly.

Opal is opal and so is the Ethiopian material. Opal body color (potch color) can vary greatly within any opal deposit. Think of Australian black opals as well as other potch colors found in Oz and elsewhere. Precious opal (with play of color) is separated from common opal (no play of color) by the size and stacking arrangement of the tiny spheres as they are deposited from water-borne silica. Random-sized spheres: no play of color. Even-sized spheres: play of color as light diffracts through spaces between them. The color(s) depend on the size of the spheres.

The technical name for thundereggs is lithophysae. They are rhyolite "bubbles" formed inside volcanic perlite flows. They can be solid inside, hollow, or filled with a variety of minerals leached by water from surrounding rocks or soils: agate, opal, calcite and others (but not obsidian aka volcanic glass). The process of formation is briefly shown here: Lithophysae This will answer at least a few of your questions. Let me know if you need more information.

 

[OpalLoverCH]

Dear Richard M.

Thank you very much for your answer.
You know really much about the subject. Nevertheless I have allready known the first part of your message, and your right !(What else).
Lithophysae are new for me, and first I have to read all about it on your link `Lithophysae`, which you gave me.
It`s very interesting. The most important questions which I have at the moment are: How the opal gets into the rhyolitic `bubbles`, especially if they aren`t hollow.
I have seen two different kinds of them: The first one was looking like originally hollow and over the time filled up with opal, sedimented inside the bubble.
The other one was looking like solid and there was opal inside too. But the opal had very nearly the same (potch) color like the surrounding (rhyolitic) mass itself, it was brown. The opal didn`t look sedimented and there was no sharp boundary around the opalised material. Also the surface of the fracture was looking the same, from the outside to the opal inside.
One more question: The color of the opal inside the thunderegg was red, orange, yellow, green and blue starting with red and ending with blue in the middle of the egg. Logically the bigger spheres must be outside and the smaller spheres inside... Big patches with play of color with all the colors of the rainbow. You can see such a patch on my picture (icon). Could this fact (red outside and blue in the middle) depend on the cooling rate of the freshly born thunderegg ? Could the spheres grow more till the end of the solidification ? Or from an other standpoint: Quicker cooling could result in more seeds of the (growing) silica spheres (till they contact each other) ? But enough foolish questions, maybe I`m totally wrong. First I have to read more about lithophysae on the link from you, maybe the answer is there ?
Thank you very much and by,

OpalLover

 

[OpalLoverCH]

..."Or from an other standpoint: Quicker cooling could result in more seeds of the (growing) silica spheres (till they contact each other) ?"...

But if the thundereggs are cooled from outside (are they ?), so why the blue color was in the middle of the piece ?

I just forgot to write this. But the theme is not so simple - isn`t it ?

Thanks and by,

OpalLoverCH

 

[Richard M.]

OL -- Many of your questions are answered in the link I provided.

1. The lithophysae form first, as is explained, in the perlite flows (which are very rich in silica).

2. Gas pressure, as explained, creates various-sized openings inside the "eggs."

3. Over hundreds of thousands or maybe millions of years water leaches silica from the perlite and surrounding soils, which is very slowly deposited inside the "eggs" as opal, agate or crystalline quartz. Heat and pressure have nothing to do with opal formation except that some of the water involved in dissolving silica from the perlite may come from hot springs.

Entire books have been written about the opal/quartz depositional process and no one has all the answers yet as far as I know. That''s one of the things that makes quartz and related silicates like opal so fascinating: they still have mysteries. That''s mainly because they''re not considered "important" enough to spend large amounts of research money on. Some types of lithophysae opal, like the exceptionally bright gems from Spencer, Idaho, show definite horizontal "water level" lines. Other opal-filled eggs I''ve mined in Oregon contain homogenous-appearing common opal, as if a grand "pastry chef" had squeezed opal "pudding" into a rocky "eclair." Australian nobbys contain similar homogenous masses of precious opal (the MOST precious opal!)

If you''re seriously interested (around $40+ postage serious) get a copy of the brand new book "Opal: the Phenomenal Gemstone," from Amazon or your regular bookseller. Not only is it magnificently illustrated but it explains in detail what is known about opal deposition and what causes the various kinds of color play. It has a short section on Ethiopian opal but it doesn''t go into much technical detail. I think most of your other questions are covered, though.

Richard M.

 

[OpalLoverCH]

I was talking about "big patches with play of color" and now I have changed my icon (avatar), so you can see the whole fragment of such an opal. It`s only a fragment, but I have seen complete peaces of that kind of `thunderegg opals` and it looks like the rhyolitic mass itself is composed of opal...
It`s not usual, but I have attached my old avatar picture.

Does anybody else know somthing about Ethiopian opals ? Please tell me !
Thanks and by,

OpalLover

 

[OpalLoverCH]

Oh hello Richard M. !

I`m sorry because I wrote my last message at nearly the same time like you, so I haven`t seen it .
It`s really great to have contact with such a knowing person like you.
Do you collect opals like me ?
I have seen the cover of this book in the web and was not sure to buy or not, but if you tell me that it`s a interesting book I will do so.
I have to finish at the moment, because it`s very late for me now (03.22 in the morning), but I will write again next time.(My problem with writing in english makes me very slow.)
So thank you very much for your interesting answers and by,

OpalLover

 

[Richard M.]

Hi,
If you have a CD disk handy, hold it under a bright light and look at the spectral colors. No single color occupies the same space, does it? The CD "rainbow" is caused by light interference and diffraction (not refraction).

As white light passes through an opal, tiny spaces between the silica spheres can cause diffraction and break the light into its component colors. Meanwhile, some light waves overlap, which reinforces some colors and cancels others. This phenomenon is called interference.

When the silica spheres are smaller than 150 nanometers or larger than 300 nm there is no play of color, resulting in potch or common opal. (Potch colors like the brown in your stone or black, pink, blue, etc. are totally unrelated to play of color and I won''t discuss that here).

When the sphere sizes are within that "precious opal zone," color play results. As simply as possible, precious opal''s color play also depends on: the diameter of the silica spheres in any given area of the opal, the angle of light directed to its surface, the uniformity and alignment of the spheres, the stone''s orientation and the angle it''s being viewed from.

Still with me? When the spheres are about 300 nm in diameter, white light is broken into all the spectral colors it contains (we used to have to memorize the mnemonic VIBGYOR in school to remind us those colors are violet, indigo, blue, green, yellow, orange and red).

When the spheres are about 150 to 200 nm diameter, only green and blue are seen. And so on. Some opals are predominantly red, others orange, green, etc. That''s all related to the size of the spheres as well as the stone''s orientation and the angle of view -- stones will often change color as they''re rotated.

When there''s an abrupt color change in an opal from the same angle of view, as in the famous patchy-colored Harlequin types, rows of silica spheres are intersecting at different angles, throwing different spectral colors.

This is the cheap "basics only" lecture and without illustrations and a lot more space it can''t answer all your questions. If you don''t want to spend $40 for the book I recommended (it''s well worth it if you''re a true opal lover!) I can also heartily recommend Fred Ward''s less expensive but truly excellent book "Opals" (Fred Ward Gem Series). When you see the illustrations and electron microscope photographs much of the above explanation will start to make sense (I hope!).

Richard M.

 

[OpalLoverCH]

Hi Richard M.

Thank you for the answers. I allready know most of the opal basics, which jou have shown me (sphere size, self assembly, size necessary for fire play, angle of view vs. light source, etc.)
But I tell you this: At your link `Lithophysae` is the part `What Are Lithophysae` and in the second picture (first foto)there are four thundereggs. It shows the various `stages` in the opening of a cavity inside the pieces. If you look at the first piece in this picture then you can imagine what kind of thunderegg I mean. I mean the ones whitout a cavity. If opal is deposited into the ones whit a cavity, then you will see something like a boundary line between the opal and the surrounding shell/mass . The (body/potch) color of the opal would be different than the color of the shell. Do you agree ?
So how do you think comes the opal inside such a peace, whitout a cavity ? And if the body color of the opal is very the same like the color of the surrounding (rhyolitic) mass itsels (whitout any boundarylines) like in the first peace of the picture in your link. How ? Water is small enough to diffund inside, but silica spheres of some hundreds of nm ?
A second thing is, that in those opals (my avatar pictures), if you turn them like a compass the blue part of a patch will stay blue and the red stays red, there is no change, so the colors stay at their points and turn the same way as you turn the stone.(No color change). Every big single patch looks like a rainbow. If the angle of the way of light is smaller (from the lamp to the stone and to the eye), all colors of that `rainbow patch` are chanching to the smaller length of light wave, like normal (red to orange, orange to yellow, etc.).
That`s because I think the spheres have to be smaller at the blue part of the color patch and the spheres at the red side of the patch have to be bigger. I know there are other opals too, but those are like that.
So how could they deposit in this mass ? And how in this special order ? I`m allready finished the lecture on your link, now I know most of the things inside the lecture, but I still haven`t found the answer to my question.
Couldn`t it be that this kind of Ethiopian opal (seems not like sedimented or `squeezed in`)is composed entirely of nanometer sized (rhyolitic or else ?) granules/spheres and that the spheres (silica ?) are precipitated in the flow of the liquid mass ? Maybe to grow on in the formed (still chewy) thundereggs, till the spheres come in contact whit each other ?
Otherwise how could one explain my observations ? Am I really wrong ? I hope that I do not stress you whit my questions. Do you allready have the following link ? Very interesting observations: http://www.bweaver.nom.sh/darwin/darwinai.htm
Thank you very much for your time and by,

OpalLover

 

[Richard M.]

Hi OL,
I''ll need some time to absorb the Darwin link (thanks). I guess I''m not really sure of what you''re referring to. I don''t see how a non-hollow nodule can have opal formation inside and rhyolite outside, at least as I understand how opal forms and if I understand your question, but I have some experience with that -- see below.

This new link on Del Norte thundereggs is the most comprehensive examination of lithyphysae formation and development I''ve encountered. DEL NORTE THUNDER EGGS Maybe parts of it (especially the few paragraphs devoted specifically to opal formation along with the various transition phases of opal) will be of help to you.

As Dan Kile says: "The only consistent aspect of the various theories for thunder-egg formation is a relative lack of either consensus or consistency." The same holds true for opal deposition as well, in my experience.

I''ve run across things in the field that still have me scratching my head in wonderment after many years. While prowling in the Idaho desert many years ago I came upon an abandoned cabin. On a window sill inside were several rhyolite nodules. I cracked one of about 3.5 inches diameter with my rock hammer. It was solid rhyolite except for a kernel of bright orange fire opal about half an inch in diameter in the exact center. It had no play of color but was as vividly-hued as the finest Mexican fire opal. There were no cracks in the nodule and I have no idea how the opal formed inside. Maybe it was the same process that has you puzzled. I spent an hour or so scouting the area, trying to find a source for the nodules, but a great abundance of rattlesnakes in that location cooled my enthusiasm for the search.

As for potch color variations I don''t know of any scientific explanations for them. I''ve read lots of speculation but nothing backed by scientific proof. Maybe someone else will chime in with explanations for all of these questions.

Richard M.

 

[OpalLoverCH]

Hi Richard M. !

One more time a bigger picture of the fragment of such an opal. You can see the beautyful patches of fireplay on it. You see the body color too. I hope that I can show pictures of entire pieces of this special type of thunderegg opal in the future. So if I will see the seller one more time and if he still has such peaces...
Thank you for everything and by,

OpalLover

 

[OpalLoverCH]

Dear Richard M.

Just one more thing. If you visit the Darwin link: The picture `No.3` (Fragment of a spherical volcanic bomb) was the one that was telling me, that my idea could be right. If the upper part of the fragment would be the interior part of the bomb and if those cellular spheres would be nanometer sized, then it could be like that type of Ethiopian opal (interior part blue, exterior part red color play). You can see that smaller spheres are in a more ordered lattice than the bigger ones. And the smaller ones are more monosized than the bigger ones. So if they would be nanometer sized...Thinking about electron micrographs of opal.
Thank you for the new link too. I think, I will need more time than you to absorb it...
Now i`ll go to sleep. Till the next time ! Thanks and by,

OpalLover