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EducationGrowing selenite crystals

31st Aug 2017 02:53 UTCJack Brooks

Hi all, since selenite is water soluble, can you grow crystals? I boiled water with coarsely powdered selenite and it did not dissolve. I just put a batch of coarse selenite bits into my mill to make an even finer powder. Should I try dissolving it in an acid? How do I go about growing selenite crystals? Thanks

31st Aug 2017 05:01 UTCDoug Daniels

To do it by dissolving gypsum in water, you'd need a heck of a lot of water.... I knew a guy in Houston that grew them by dissolving the gypsum in hydrochloric acid (muriatic acid, pool acid), then letting the solution evaporate. Made some crystals that were at least an inch in length. Haven't tried it myself, but worth a try. And, if you try this - keep the solution outside! You don't want any evaporating acid fumes (yes, that is the reaction...) attacking anything in your house/apartment/whatever.

31st Aug 2017 05:09 UTCFranz Bernhard Expert

It would be a little bit tricky. Solubility does not change much with temperature, and its maximum is at ca. 40°C:

http://hydro-land.com/e/ligne-en/doc/p/Solubilite_CaSO4-2H2O.jpg

However, dissolution rate will increase significantly with temperature.

Acid is not a good option, but some "impurites" like NaCl will increase general gypsum solubility 2-3 fold.

I have not tried it, but maybe you can work with a temperature gradient, keeping one side of a vessel with some solid gypsum at ca. 40°C, the other side at a temperature as low as possible. In theory, some gypsum should be transfered via dissolution/precipitation from the warmer to the cooler side. But it will take time (months, perhaps years) to see significant crystals growing, because of the low solubility.

Maybe someone has experience with growing gypsum crystals?

Franz Bernhard

31st Aug 2017 08:51 UTCDavid Baldwin

Gypsum crystals in caves such as the Naica mine or Lechuguilla are attributed to sulphuric acid dissolution and in the case of Naica, an ideal growing temperature of 58°C, but as Franz points out, time could be an issue.

31st Aug 2017 11:36 UTCGregg Little 🌟

I frequently grow gypsum crystals as a test for anhydrite in drill samples. As noted above, I put the test sample in 10% HCl and heat. Upon cooling the needles of gypsum/selenite form over ten minutes or so. They are micro in size but this technique does work. Larger crystals probably just take more time, material and patience. If you use the micro crystals as seed crystals might you be able to enhance the process for larger crystals?

31st Aug 2017 11:51 UTCBen Grguric Expert

According to Donnelly (1967) tiny twinned gypsum crystals are readily grown by putting a drop of seawater on a microscope slide and letting it evaporate. This paper is available here http://www.minsocam.org/msa/collectors_corner/amtoc/toc1967.htm free of charge (pg 1-12). I've never tried it myself.

31st Aug 2017 12:01 UTCReiner Mielke Expert

Seems to me the easiest way would be to dissolve calcite in sulphuric acid untill it is neutralized and then let the solution slowly evaporate. That is basically how gypsum grows on weathered sulphide deposits.

31st Aug 2017 12:03 UTCFranz Bernhard Expert

Have not found a diagram for gypsum-HCl, but one for gypsum-H2SO4. Strong increase in solubility and strong dependence of solubility on temperature with increasing H2SO4-concentration!

https://qph.ec.quoracdn.net/main-qimg-a7012168108be07a68f04936a48b383a

Growing gypsum crystals in a temperature gradient or by cooling would be much more efficient in a (strongly) acidic medium.

Franz Bernhard

19th Jun 2023 23:17 UTCRocki Wingard

I know this is old, but I never get to talk chemistry anymore!

I think starting with seed crystals placed in a solution of magnesium sulfate (Epsom salt) and slowly adding calcium chloride and/or calcium hydroxide is a better choice than jumping straight to sulfuric acid.

Sulfuric acid has become much more difficult to buy in recent years, and it's very dangerous. In my method, the calcium will displace the magnesium, forming gypsum which will want to precipitate out of solution. At the right temperature, hopefully it will form onto the existing crystals.

Another option is to wet the crystals with the magnesium sulfate solution, then directly apply the dilute calcium chloride. I may try this to produce some alabaster.

31st Aug 2017 22:05 UTCDana Morong

Years ago, I set up conditions to grow tiny crystals, including gypsum (actually synthetic gypsum, as a mineral is only natural), by the silica gel method. I had first read of this in an old issue of Scientific American (March 1962) in its Amateur Scientist column, "Growing Crystals in Silica Gel mimics natural mineralization" by Walter R. Averett (v.206, 155-162). This got me interested, and I looked up a book in the nearby college library, its Chemistry branch, and found a book by Heinz K. Henisch (1970) Crystal Growth in Gels (his 1988 edition is Crystals in Gels and Liesegang Rings). I thought I'd try a few easier ones. I got [the synthetic analogs of] gypsum, and brushite, and a few others, in micro size (about a millimeter or two, usually, because I used small amounts of reagents, and when the reagents ran out, the crystals stopped growing). Sometimes I made up a solution of the gel, poured it into a U-shaped tube, let it set up, then put each reagent, in solution, on either side. Then they slowly permeated the gel and met in the middle to form crystals (the gel as a medium to prevent fast crystallization and hence powder - I wanted crystals big enough to see crystal habits under the low-power microscope). Or other recipes call for mixing one of the reagents into the gel solution, letting it set up in the bottom of a regular test tube, then when the gel is 'set' one adds the other reagent on top.

I got some mounted (usually I would put a thin layer of glue on black paper, then sprinkle the dried crystals onto it, they'd stick and the glue would dry, and I would put it into a little m/m box and label it... I still got some, the typical gypsum penetration twins, 1-2 mm in size.

I kept the recipes (sometimes I tried for something and came up with something else instead), and later made up a sort of list of articles about this. Later I went to the chemistry library (nearby college) and looked up articles on the method in Journal of Crystal Growth. There are a couple (in 1971 and in 1974) on calcium sulfate dihydrate crystals by the silica gel method. have a list of equipment, ending with slide rule and notebook with pencil (for writing stuff down at the time). It was interesting.

Have not actually tried to grow them by water solution, but have seen where a chunk of gypsum lain in the soil for some years (in a rockhound's waste pile) had its surface apparently dissolve a bit and recrystallize, and once at an old gypsum mine in N.B. (Canada) seen where rain had partly dissolved some gypsum, making tiny, nearly microscopic, rills in it, a most interesting pattern, and reminiscent of Rillenstein in limestone (found a copy of an article on that). And of course specimens of gypsum I have left outdoors for a long time, in the rain and sun, have weathered.

1st Sep 2017 04:08 UTCGregg Little 🌟

Greetings Dana;

Last year I was poking around the gypsum docks at Hansport, Nova Scotia and musing at all the interesting weathered patterns in the gypsum rip-rap armoring the shore. Depending on the original texture (chicken wire, bedded, massive, etc.) the solution patterns were highly varied and "abstract". Never though of looking underneath for re-precipitated crystals. I just assumed the solution would have leaked off into the ground.

"Rillenstein", new term to me and I like it. Sort of like Frankenstein erosion patterns. Some of the rock had bizarre ragged shapes.

Gregg

1st Sep 2017 08:32 UTCUwe Kolitsch Manager

Rille (German) = groove, crease, furrow

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