FMF - Friends of Minerals Forum, discussion and message board

[Ru Smith]

Greetings all.

What do you think are the controls on selective etching of crystal faces? For example, the attached fluorite twin shows tiny etch pits on the crystal faces facing left in the image, whereas the front and right-facing faces are pristine (though the edges show some etching). The crystal is internally completely transparent and largely flawless. There are some more images of this specimen here:
https://www.pbase.com/hajar/british_minerals
(link normalized by FMF)

Thanks.

[Matt_Zukowski]

Selective etching is often seen on the faces of different forms, which is caused by differences in how the faces of each form are favored by different physiochemical regimes (chemical composition of the solvent and the already formed xtal, temperature, redox potential, pressure, etc). In this case, however, you seem to be presenting an example of selective etching on different faces of the same form, depending upon their orientation in the pocket. I can think of two mechanisms that might produce this result. The first is when xtal faces facing upwards in a pocket are covered with later precipitates that form above the xtal, fall through the solution, and settle on the xtal. The faces covered by this precipitate can be chemical altered (or merely shielded), and later may be more or less likely to experience dissolution. The second is where a solution moving through a pocket causes some sort of physiochemical gradient in the pocket such as minute differences of pressure or chemistry on the upstream and downstream side of xtals.

[Jordi Fabre]

Hi Ru, welcome to FMF

Let me just add to the magisterial comment of Matt that Hilton Mine seems to had a very stable period of formation because as far as I know the etched crystals from there (selective or no) are quite scarce.

[Ru Smith]

Many thanks Matt,

Are you aware of any case studies demonstrating your second idea (upstream-downstream effects relative to a moving fluid)? Could another possibility be that pocket clay covering protected some faces?

The etch pits are quite beautiful up close - arrays of octahedral depressions. I just took a couple of photos of these.

I've seen many Hilton fluorites showing etching Jordi, but my favourites are those limpid isolated twins sitting on top of an earlier generation of cubes.

[gemlover]

Some references for Mass Transfer in rocks. Many processes affect this, porosity, advection, diffusion, temperature and pressure to name some.

Bear, J.: Dynamics of Fluids in Porous Media, Elsevier, New York, p. 764, 1972

Colman, S.M. and D.P. Dethier, (eds): Rates of Chemical Weathering of Rocks and Minerals, Academic Press, p. 603, 1986

Crank, J.: The Mathematics of Diffusion, 2nd Ed., Clarendon Press, Oxford, p. 410, 1983

Henderson, P.: Inorganic Geochemistry, Pergamon Press, p. 353, 1982

Hofman, A. W., et al (eds): Geochemical Transport and Kinetics, Carnegie Institution of Washington, p. 353, 1974

Lasaga, A.C. and R.J. Kirkpatrick (edsZ): Kinetics of Geochemical Processes, Reviews in Mineralogy, vol. 8, Mineralogical Society of America, 398 pp., 1981

Stumm, W., and J.J. Morgan: Aquatic Chemistry 2nd ed, Wiley, p. 780, 1981

Sverdup, H.U.: The Kinetics of Base Cation Release Due to Chemical Weathering, Lund University Press, p. 246, 1990

Walther, J.V., and B.J. Wood: "Mineral-fluid reaction rates," in Fluid-Rock Interactions during Metamorphism, Springer Verlag, New York, pp. 194-212, 1986

Sorry these are somewhat older references, but I graduated almost 50 years ago, and I have not kept up to date totally in the literature.

John

[Ru Smith]

Thanks John,

A little light bedtime reading!

[Matt_Zukowski]

You should know that I don’t claim any measure of expertise in this area. Also, I do not know of any specific studies showing selective etching in fluorite caused by fluids moving through a pocket. I do believe, however, that micro environments can form in a moving flow field, and that this can influence xtal growth and development. For example, I found some papers from the material science literature that talk about such a mechanism (e.g. Stanislas Sizaret, Ivan Fedioun, Luc Barbanson, Yan Chen, Crystallisation in flow Part II: Modelling crystal growth kinetics controlled by boundary layer thickness), but I don’t know if any of this is relevant to your situation. A good book on crystal growth and dissolution mechanisms is Sunagawa (2005) Crystals: Growth, Morphology and Perfection. On page 237, when describing the growth of one beryl xtal in a pegmatite, Sunagawa writes, “From the observation that the thickness of growth sectors of crystallographically equivalent faces are different in different directions, we see that there was anisotropic flow of the mother liquid and that the flow directions varied during the growth process.” He goes on to say that this lead to different dislocation densities (and other differences) on different faces. I do not know if changes in the density of dislocation densities or any other differences on different faces of the same form could lead to the large differences in etching potential that your specimen exhibits, but it seems at least remotely possible.

[Ru Smith]

Many thanks Matt. It seems a plausible explanation to me too. I wonder how fast these fluids move?

Best regards

[Ru Smith]

I just took another photo of this twin. I hadn't noticed the blue growth lines before.