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Techniques for CollectorsCleaning Hour Glass Selenite crystals found in Oklahoma?

9th Jun 2016 20:17 UTCScott DeLano

Hello!

I was out at the Great Salt Plains selenite location in Oklahoma this past weekend. I was able to extract a few nice clusters of selenite crystals and some nice "blade" crystals from there. There appears to be a bit of the red sandy grit on some of the crystals, and I am wondering if there is a good way to clean them up (possibly removing a bit of that grit) without damaging them? Any ideas?

TIA,

Scott

9th Jun 2016 20:45 UTCBob Harman

Since gypsum, including selenite variety, is quite sensitive to water, care must be observed. If it is just sand or a bit of grit on the surface of the crystals, maybe a terrycloth hand towel lightly moistened with a dip in 100% acetone or 100% alcohol (absolute alcohol) might work. Just briefly and gently wipe the moistened towel over the crystal surface. Remember NOT to use water if at all possible (or just a VERY VERY brief wipe followed immediately by gentle drying might also be acceptable. If you have found several or more examples, you might try to experiment first with your lowest end example. CHEERS.....BOB

10th Jun 2016 02:14 UTCDoug Daniels

Definitely avoid using hydrochloric acid (muriatic acid) - it will dissolve the gypsum.

10th Jun 2016 02:23 UTCAlfredo Petrov Manager

I'd use distilled water and a soft toothbrush. As Bob mentioned, gypsum is slightly soluble in water, so a tiny tiny bit of your gypsum will dissolve away, but if you're only working on it in water for a few seconds (which is probably all that's needed), it would take an electron microscope to observe any etching on the surface, so no worries.

10th Jun 2016 04:38 UTCDoug Daniels

Afredo - wouldn't "hard" water be better? It's already saturated with the calcium salts, so should dissolve even less than distilled water would. (I know, I'm pushin' it....and, as you said, you're only using a tiny bit of water anyway).

10th Jun 2016 04:57 UTCAlfredo Petrov Manager

You could be right, Doug, although I think there might be some small chance of added minerals deposited on the surface when the specimen is dried, leading to dulling of the luster. But that's just a supposition; it might not cause any problems at all.

10th Jun 2016 15:52 UTCSteve Hardinger 🌟 Expert

Funny thing about gypsum -- especially compared to many other inorganics -- is its solubility. In most cases solubility increases with increasing temperature. In a few cases solubility actually decreases with increasing temperature. Gypsum on the other hand peaks at about 40 C, and drops off on either side. Thus if you are using water to clean gypsum (not a problem if contact is brief as mentioned above) best to use cold water instead of warm water.

I've used the moist toothbrush method myself many times, with good success.However to minimize (or maybe even prevent) the need for water at all, try a gentle cleaning with 'canned air' first. This may remove most/all of the nonincluded sand particles. Sand that included within the gypsum crystals isn't going to budge (without destroying the gypsum) regardless of your cleaning method.

3rd Jul 2016 16:35 UTCJonathan Woolley

I'm a little late to this thread, but I can share my experiences cleaning the Salt Plains selenite crystals. They are extremely sensitive to water (including distilled water), and any attempt I've made to clean them that way can result in a noticeable dulling of the luster. Pure speculation, but maybe this has something to do with the fact that the crystals are actively growing in the saturated groundwater and maybe there are still microscopic growth irregularities? I'm sure someone more qualified can comment on that speculation.

My recommendation is commercial "rubbing alcohol" (70% Isopropyl alcohol) available from any drug store. I have used this with great success on crystals from the OK Salt Plains with no apparent dulling. Not to mention, it is a very cost-friendly solution.

17th Dec 2018 22:22 UTCMark James de Bruijn

I boiled small selenite stick for 28 hours and it did not dissolve, or disintegrate in the least. The myth that selenite dissolves in water is unfounded as it would take forever. However, if the water is very acidic, perhaps. People say that you should not put is any where near water, and we clean, and remove burrs from selenite by putting in a big sink of hat water, and coming back to it the next day to scrub with a stiff fiber brush.

I do want to try different acids, mixed with water to dissolve it, as i want to try an experiment.

18th Dec 2018 00:04 UTCDoug Daniels

Selenite (gypsum) is unlikely to dissolve in boiling water - it is more soluble in cold water. Granted, it takes an awful lot of water to dissole even a small specimen.I remember aback in college, a young lad collected a chunk of selenite, but it was a bit dirty. So, he put in the sink in our restroom area, turned the cold water on it, and left it overnight. The next morning, wasn't much left. Now whether it dissolved or was worn away, I don't know.

18th Dec 2018 02:46 UTCJonathan Woolley

Dissolution of an entire crystal might take longer, but (perhaps especially for actively growing crystals such as can be collected at the Great Salt Plains Wildlife Refuge here in Oklahoma) it doesn't take nearly as long to simply damage the luster.

18th Dec 2018 03:59 UTCThomas Lühr Expert

One liter of water solves about two grams of gypsum. That is quite a lot. The solubility is even much better if the water is salty. The maximum of solubility in pure water is at about 30°C. I don't know for if it is the same for salt water (probably not).

Sure this solubility is enaugh to make crystals dull. To avoid that, for cleaning can be used either a liquid the does not solve gypsum (e.g. alcohol) or use a saturated solution of gypsum in water, that can not solve further gypsum.

Usually the second option works better, because water is a better cleaning agent than alcohol and even more than hydrocarbons due to its strong polarity.

Gypsum powder from the construction market (not exactly the same like selenite) can be used to prepaire the saturated solution.

18th Dec 2018 04:43 UTCDoug Daniels

Yes Thomas. Gypsum powder can be used to make a gypsum-saturated solution. Here in the U.S., the powder can likely be found at any local craft store - sold as Plaster of Paris. Granted, it is dehydrated gypsum, but put a little into a large enough amount of water, and you'll end up with a saturated solution. Or, if you have some ugly gypsum that you don't know what to do with.......

And, if you really want to experiment with acids, note that hydrochloric acid (muriatic acid, pool acid) WILL dissolve gypsum. Don't know about any others.

18th Dec 2018 05:26 UTCThomas Lühr Expert

"And, if you really want to experiment with acids, note that hydrochloric acid (muriatic acid, pool acid) WILL dissolve gypsum. Don't know about any others."

I think the solubility increases not due to the acidity of the hydrochloric acid but because of the chloride ion. Thus, the effect should be about the same as with ordinary salt.

Here will be a large field of experimenting :)

6th Jun 2019 02:09 UTCCatherine Costello

A sizable piece of selenite will not dissolve in water over night or over several days. Yes, gypsum is very water soluble, UNLESS the water is already over saturated with gypsum. As in the case of the 'Cave of the Giant Crystals' under the Peñoles Mine near Naica, Mexico. This applies to the hourglass crystals of the Great Salt Plains in Oklahoma also. My guess is your selenite stick is actually gypsum 'Satin Spar' as Selenite does not grow in sticks. Your stiff brush would horribly scratch the face of real selenite.

6th Jun 2019 02:11 UTCCatherine Costello

To clean up my hourglass selenite crystals from the Great Salt Plains, I used my kitchen sprayer on the larger ones to wash off the loose sand. Smaller, more delicate ones I just let the water trickle from the faucet over them while I gently rubbed the surface. The thinner ones will start to disintegrate before your eyes if you're not careful. Sometimes it is best to leave the grain of sand or pebble that is partially embedded in the crystal blade. I call it character. Good Luck!

6th Jun 2019 10:54 UTCTimothy Greenland

If you want to test the solubility of gypsum, put a piece in the tank of your water closet - then look for it after a week or two of use. It will probably be gone or much smaller...

Cheers

Tim

7th Jun 2019 15:52 UTCDana Morong

I think it might depend on the water you use to dissolve it. The tap water where I live is so 'hard' (so much calcium in it) that it leaves spots after a shower, and every now and then I have removed shower head and soaked it in vinegar for a few hours, to try get the crusts off. I don't know how that would work with gypsum. However, I have a piece of gypsum that I collected some years ago, now in the yard, under the dripline of the roof, and it appears to be slowly eroding, making an interesting effect. I suspect that the rainwater has a lot less dissolved ions in it than the tap-water.

8th Jun 2019 02:02 UTCKeith Compton 🌟 Manager

Here is an earlier thread some time back.

https://www.mindat.org/forum.php?read,19,112465,112763

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