FMF - Friends of Minerals Forum, discussion and message board

[Carrington]

Hello,
A few months ago I asked people on mindat on how floater crystals form. Although not the same topic, I am now asking if anyone knows a book or article of some kind which can describe the process of etching of crystals in depth. I do know that solutions act with the mineral, but is there more reasoning as to how the solutions come into contact etc.? Any information on etching Is helpful. Thanks.

[Jordi Fabre]

We have two stupendous threads about floater crystals:

- "What are "floaters?" -> https://www.mineral-forum.com/message-board/viewtopic.php?t=195
- "Article in M. Record about the John White single crystal collection" -> https://www.mineral-forum.com/message-board/viewtopic.php?t=628

but as far as I can remember not so much about etched crystals. Anyway, you have some useful read here:

- Aquamarine and Etching -> https://www.mineral-forum.com/message-board/viewtopic.php?t=997
- Inclusions of spiral or helical growth and screw dislocations -> https://www.mineral-forum.com/message-board/viewtopic.php?t=1337

[Peter]

When minerals crystallise in a pocket their formation and possible etching will depend on the various conditions that occur during the history of pocket formation and after as well.

If you imagine crystallisation of a "pegmatite solution bubble" inside the mother granite there will be various stages of minerals crystallising from the solution from the outer walls to the very center. If the specific conbination of emplacement and solution composition will create a pocket pegmatite, even during the last few hundred degrees temperature drop and span of chemical composition of the solutions still in the pocket could create various alternating crystallisation and etching. I have briefly described the Volodarsk chamber pegmatites in the Mineralogical Record a few years ago.

The outer shell of the pocket pegmatites may have started to crystallise at temperatures below 700C, perhaps around 650 C and inside the open pockets minerals have crystallised from obviously much lower temperature (some 350/400C and the final stages around 100C). During this crystallisation the rest solutions composition will change depending on what mineral is crystallising, it means also that the pH will change beeing acidic to neutral to alcalic and so on. Each pH and the reason for this pH (i.e. chemical component) will etch various minerals, their various faces to various degrees.

Please note also that there may be booth a liquid and a vapor face inside the pocket or one of them, and this at various stages. A liquid may reach only up to a certain level in the pocket., Crystals may be partly or fully protected (depending where they occur in the pocket) by protecting clay minerals etc......

In Volodarsk for instance I found huge casts of large beryls inside clay where the remnant tiny bit of beryl was a fraction (a % or a few % or so of original crystal)

The shape of the etching will depend on what face the etching occurs in and the etching media. This etching is typical for the edges. Hexagonal etch pattern typical for the pinakoid (flat top) termination. A multitude of forms for other faces, as we are "attacking" the beryl molecule from different directions.

John Sinkankas book "Emerald and other beryls" describe some etchings of beryl.
In the chapter checmical and physical properties he summaries many good references and research into beyl etching, also laboratory trials and study of natural crystals.

One researcher (Honess) obtained "satisfactory" etch figures on beryls in 15 seconds by immersing them in moltensodium hydroxide where boling hydrofloric acid for an hour failed to produce the slightest trace of etch figures (page 277 and 268).
I have many examples of interesting groths and etch figures.
Best regards
Peter

[Carrington]

Thank you Jordi For the references! Very helpful. And Also to you Peter for some interesting information.

[Antonio Alcaide]

By chance I am reading now the book "Mineralogy for Amateurs" by John Sinkankas. You can read on it a short point about etching, that is supposed not to be as deep as the studies on beryl commented by Peter in his last post. The author explains the proccess adding several examples and a figure.

If you want, I can scan the pages (only two) and send them to you via pm.

Regards

[Carrington]

Hello Antonio, Thanks, but I own that book. I have read the information he offers, and it is helpful.