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What's the deal with etched minerals?

Posted by John Stolz  
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John Stolz August 29, 2012 09:32AM
Hey all,

I have a general question about why etched minerals like beryls and tourmalines are so desireable. I've seen a few--mostly in pictures--and I don't understand why they seem to be preferred over non-etched specimens.

John
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Maggie Wilson August 29, 2012 01:04PM
Very timely post, John - I was wondering about the etched Brazilian beryls just yesterday - more along the line of how is it that the very coarsely etched crystals still maintain a high lustre after they have been etched. I assume that there are different degrees and/or mechanics of etching? And I suppose the attraction is in the gem potential? All guesses - would love to know more.

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Robert Simonoff August 29, 2012 02:36PM
And I would love to know what naturally occurring chemicals are in the pockets that cause this etching. I have seen many guesses, but no one saying that they know the answer

Bob
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Mario Pauwels August 29, 2012 02:47PM
I dont think that you can say that generally all "etched crystals" of Aquamarine or Heliodor are prefered over the non-etched rough gem crystals, like always it simply depends on the specimen.
Like Maggy said, the "etched faces" of a rough gem crystal have almost always a high luster and show strong reflections, while most "non etched" rough gem crystals not necessarely have that high luster and missing the typical strong "window" reflections on their surface to. Also the typical appealling and repeating "window patern" is something that is highly appreciated on "etched " gem crystals.

Best regards,
Mario Pauwels
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Owen Lewis (2) August 29, 2012 03:34PM
It is interesting, isn't it? Your Beryl does not look etched to me Maggie, but looks to have been fractured, naturally or artificially to create that overall shape and appearance. Sometimes I find it hard to be certain what are high points left after a general etching away of the surrounding area (as buttes are left standing proud of a desert floor) and what are growth structures on a generally planar surface. Then are etch pits indeed etched pits or are they sometimes small points of arrested/retarded crystal growth? Sometimes and through a microscope one seems to find a combination of these conditions on different crystal planes. Here's pics of what I take to be a cubo-hexahedral colourless Fluorite that shows this. This crystal is approx 2.5cm in diameter.



But, to address John's point directly, if the plane(s) of a termination cannot (even indistinctly) be seen on a crystal as presented, then to say that there is such a termination (or broken end or contact area) is simply wishful thinking. One simply does not know what may once have been there.



Edited 1 time(s). Last edit at 08/29/2012 03:36PM by Owen Lewis (2).
open | download - Fluorite 3-08a.JPG (899.3 KB)
open | download - Fluorite 3-03a(1).jpg (883.5 KB)
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Alex Homenuke August 29, 2012 04:18PM
One form of etching would be resorbing of a mineral into formation solution as the equilibrium changes. Some of the more delightful "etchings" are the MSH clear quartz specimens.
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John Stolz August 29, 2012 11:31PM
We're kind of moving off tangent, but the subject is still interesting--I found some good information on the subject at http://www.mineral-forum.com/message-board/viewtopic.php?t=997 along with some excellent pictures (sorry Maggie, my brain just couldn't process the picture, I just can't see what it is)

Another mystery for me that's associated with the subject at hand is how some crystals can show frosting of preferential faces. Owen's pictures kind of show this preferential etching of faces, but the specimen I have--a pakistani aqua--has frosting only of the faces that aren't orthogonal to one of the axes and absolutely no frosting on the remaining faces. I can't visualize a process that would do that, and the internet is of limited use for any kind of plain-english explanation.

@Alex: what does MSH mean?

Regards, John
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Stephanie Martin August 29, 2012 11:52PM
John, MSH is an acronym for our famous locality:

Mont Saint-Hilaire, Rouville RCM, Montérégie, Québec, Canada

regards,
stephanie :-)
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Owen Lewis (2) August 30, 2012 02:56AM
John, of the three planes partially shown in the second shot, the face to the left has mainly pits with some outgrowths, The centre-right face is almost optically flat (I actually took an RI reading off it with a little finagling) but it also has a few outgrowths. The face at the top seems to me to have a very heavy crop of almost wart-like outgrowths.

I have a strong impression that the 'outgrowths' are simply that and do not result from etching away of the rest of the surface to leave the 'outgrowths' standing proud. In the case of the almost perfectly flat face, I have never seen any etching effect that could do that. In the case of the pits, yes, etching is a possibility - but why on only the one of the three faces? Given the general present of 'outgrowth' features, is it not at least plausible that the pits might be caused not by etching by some very local effect that arrested/retards the crystal growth to create the pits as the rest of the crystal surface around then grew at a faster rate? A sort of negative growth?

Like others, I feel that there are far more questions surrounding this growth/etching phenomenon than there are sufficient answers to. It would indeed be nice to learn more.
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Anonymous User August 30, 2012 03:10AM
As a gem crystal collector, I don't prefer etched specimens. I'd rather have unetched, lustrous, and otherwise perfect (and floater). But I'd take etched and lustrous over a dull unetched piece.

As far as the etching solution, I have heard that solutions rich in flourine ion would do it. Aren't these commonly what help to form the pegmatite gem minerals in the first place?
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Kelly Nash August 30, 2012 03:55PM
The subject of different etching features on different faces was discussed here a couple months back: "Frosted Calcite" , basically different crystal faces (not related by symmetry) have different hardness's and reactivities.
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Ron Layton August 30, 2012 05:07PM
I own a few gem crystal T/N's and I prefer the etched ones for their aesthetic appearance. One in particular is a Petalite from Mogok township, Burma quite similar to this one:petalite. I wonder if there was Fluorine in the solution that caused this degree of deep etching? I have collected Beryl crystals on Mt. Antero and I've found both etched and "regular" crystals within the same area. Also Topaz from Thomas Range shows some interesting etching. I wonder, is there more than one cause for this? It doesn't seem likely that Fluorine is responsible for all etching.
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John Stolz August 30, 2012 06:37PM
@Ken--well I'm glad to see I'm not the only gem mineral collector that prefers unetched. Wait--that makes us competitors! ;) And if you skim the the link I gave, you'll see that there's some discussion on HF as a common pegmatite etching acid.

@Kelly--thanks for that--I never considered anisotropy! My bad!

That's quite a mouthful Steph--no wonder you abbreviate it!

John



Edited 1 time(s). Last edit at 09/01/2012 09:56AM by John Stolz.
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Ryan L. Bowling August 31, 2012 12:51AM
John,

Sharp crystals on matrix are the most desired, I would say in any species, while single crystals mostly fall into a second tier.

Single crystals, either etched or not, have to have special properties for me to like them. Most important in my mind is color, the perfect hue can make any crystal exceptional. Luster is equally important. Get the two together, and one usually has an exceptional gem crystal.

Etched beryl crystals many times have exceptional growth patterns, almost like finding oneself lost in a crystal maze.

I myself, have three etched gem crystals, all Brazilian. I love them all, great color/growth patterns, and luster. I would take a fine colored etched Brazilian aqua, over a pale colored Pakistan aqua on matrix, any day of the week.

All the best,

Ryan
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John Stolz August 31, 2012 03:58PM
Ryan,

I agree with you on the importance of color and luster--that's kind of what prompted my question: I saw an exquisite picture of an etched aquamarine here http://www.mindat.org/mesg-6-269667.html posted by Jason Barrett and I got to wondering if the etching didn't actually enhance the color/luster over what it would present as if unetched... and yet, there's the question of form in addition to color and texture--and I can't help but question the aesthetics of an etched crystal v. an unetched one, all things equal.

I need to spend some time looking at the real things instead of pictures--often I have trouble understanding what the photo is showing.

John
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Ronald J. Pellar August 31, 2012 05:40PM
I like to collect mineral specimens! An etched xtal, to me, is not a mineral specimen. Likewise, rare minerals that are blobs, especially small ones with paper arrows pointing to them (what I affectionately refer to as "arrowite" speciment) are not mineral specimens as far as I am concerned. A good mineral specimen should show some morphology.

I have judged many competition cases and I generally discount the quality score for specimens lacking any morphology, including etched xtals. The less morphology present the more the discount.

To me, beautifully colored, severely etched "xtals" are strictly gem material.
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John Stolz August 31, 2012 07:54PM
Bingo! Give that man a cigar!
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Alfredo Petrov August 31, 2012 08:03PM
And while Ron is enjoying your fine cigar, John, help yourself to his platinum nugget, as he obviously doesn't appreciate it :)-D
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William C. van Laer August 31, 2012 11:43PM
Etching is a phenomenon that is probably due more to equilibrium than to dissolution...John Sampson White argued this point in an issue of Rocks & Minerals a few years ago, but was unconvincing since the mechanism was more likely due to instability within the system of crystallization in the pocket.The forces that create a crystal's development are controlled by MANY factors, including concentration, Eh, pH, temperature, pressure, activity, free energy, all driving the reaction to go forward to favor the products of that reaction. It only takes a little change in some of these systems to reverse the direction so that the reactants are favored, which begins the disintegration of the mineral. depending on the conditions, that reaction might go to completion, ending up with no trace of the original mineral. This often results in a cast where the mineral once sat, and in some cases (i.e., beryl to bertrandite) a new mineral will form from the original reactants.

Since some minerals like tourmaline seem to do this often (plus beryl, topaz, and many others) there is a clear tendency for this to occur. In the case of tourmaline, where different colors withing thew same crystal represent differing stabilities, the earlier phases are less stable and tend to "etch" first, hence the irregularity of the example that Dr. White gives. It is NOT due to the deepth of immersion in the pocket fluid, but the differing stabilities of the two phases of tourmaline growth.

Often what is referred to as "etched" crystals are just growth--reverse growth phases repeated, as the line between stability and instability is crossed again and again (a good example of this is the spessartine garnets found recently in Brazil, along with those found elsewhere in pegmatite pockets like the Little Three Mine in California).

It is clear that many etched crystals are less that aesthetic, but there are those that break that rule!

Chris

William C. (CHRIS) van Laer: "I'm using the chicken to measure it..."
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Owen Lewis (2) August 31, 2012 11:59PM
Thanks Chris. That's a helpful clarification of what I think I've been stumbling towards.
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Ralph Bottrill September 01, 2012 01:08AM
Etching is often ascribed to HF, usually without much evidence. I often wonder if boron has an effect, the pegmatites where you commonly see etching are often B-rich and it makes a great flux for dissolving silicates and most other minerals in the lab.

Regards,
Ralph
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Roy May 15, 2015 05:16PM
Hello,

although the thread is not a new one, I hope someone can help.

Here a picture of topaz:

http://www.meelis-bluetopaz.com/about-topaz.html

(bottom right)

Could this be an etched topaz crystal?

Or are there naturally occuring topaz crystals with such crystal faces?

Kind regards,

Roy
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Rob Woodside May 15, 2015 05:33PM
Certainly topaz naturally etch making quite fantastic forms. So does spodumene.
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Roy May 15, 2015 05:57PM
Thank you, Rob.

I just found an article about etching of topaz by use of potassium hydroxide or a mixture of potassium
bisulfate and powdered fluorite (The Etching Figures of Topaz, Am. Miner., Vol. 6, No. 4)
But this was done only for a short time to study symmetry features.

I think one could expect the same strange morphology after prolonged periods of etching (?)


Roy
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