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Mineralogical ClassificationSelenite vs Satin Spar

28th Jul 2020 07:05 UTCAngel White

Hi. Sorry if this is the wrong section for this but I am looking for some information on selenite, or selenite vs satin spar. I couldn't find anything on this site by searching.

I have done a little research on the net, seeing photos and reading stuff about the naica caves in Mexico where selenite, or satin spar is mined. Also shows satin spar growing along with it from the same matrix?

Is it the same mineral, just different growth habits? Or is it a marketing term?

28th Jul 2020 07:24 UTCKeith Compton 🌟 Manager

If you enter selenite or enter satin spar in the bottom search engine (mineral box) at the foot of the page - you will get a search result.

You can also enter either name in the search box on the top menu bar - Search mindat.org and you will also get the same results.

Don't use the search box under the discussion heading - that is only for discussion messages

28th Jul 2020 08:11 UTCBob Harman

I consider selenite and satin spar to be accepted varietal names for the mineral Gypsum (calcium sulfate dihydrate: CaSO4 . 2H2O).

Selenite is transparent and satin spar may be opaque or translucent with a white fibrous or "satiny" appearance.

Here is a self-collected example of both from a 5 cm gypsum seam in a quarry in Lawrence County Indiana. The 13 cm selenite example contains sand or grit inclusions, but the black background is clearly visible thru the specimen. Note the opaque white fibrous "satiny" appearance of the 5 cm satin spar specimen , found in another area of the same seam in the quarry.

There are 2 underground gypsum mines within several miles of this quarry.

I have previously posted this same photo on a couple of other related threads in the past. BOB

28th Jul 2020 09:34 UTCPeter Nancarrow 🌟 Expert

Selenite

Naica, Saucillo Municipality, Chihuahua, Mexico

Angel,

As Bob said, they are both varieties of gypsum; selenite being the name given to limpid crystals, (see photo above), and satin spar being a mass of fine fibrous crystals in parallel growth. (see my next post)

Pete N.

28th Jul 2020 09:34 UTCPeter Nancarrow 🌟 Expert

Satin Spar Gypsum

Saurieši Quarry, Stopiņi Parish, Ropaži Municipality, Vidzeme Region, Latvia

Satin spar:

28th Jul 2020 10:28 UTCHerwig Pelckmans

Excellent job, Peter, very educational.

Thanks!

Cheers, Herwig

ACAM & MKA (Belgium)

28th Jul 2020 18:15 UTCRolf Luetcke Expert

Peter did a wonderful job of giving the details of the varietal names for them. I have often tried to get people to fix the "selenite" use in our local area. That name gets hung onto the gypsum roses that are completely opaque and the term is just wrong but it is hard to show what belongs under what variety, Peter you showed it very well. I was going to do the same but you already did it. Only one left out is the gypsum that is not clear that is often called selenite.

28th Jul 2020 18:32 UTCRolf Luetcke Expert

Thought I add the one I often see that uses "selenite" in the naming and is not correct. The proper usage here is just Gypsum.

28th Jul 2020 18:37 UTCRolf Luetcke Expert

Just looked at the Saint David, Arizona page and the ones that had said "selenite" rose have been fixed, thank you to those who fixed them.

28th Jul 2020 22:44 UTCAngel White

Thank you for al the info! That last photo I've ever only seen referred to as desert rose. But yeah it's just gypsum. Also with the different classifications, I just always considered it different varieties of the same thing. Like calcite and optical spar?

29th Jul 2020 04:58 UTCHerwig Pelckmans

Correct. In most cases, desert roses do have a lot of gypsum (besides the included sand grains). In rare occasions it is barite instead of gypsum. Holding a specimen will tell the difference, barite being much heavier (hence the name!).

Don't forget that satin spar can also refer to a "silky calcite" or "silky aragonite" specimen.

See https://www.mindat.org/min-8573.html

That's why we prefer to use the international accepted mineral names, like gypsum, because those names stand for a well defined substance, and fortunately they are limited to 5606 different ones (today on mindat).

The exact number of "varietal names" as we call them (like selenite, satin spar, alabaster, and so on) is unknown, but I am guessing at least ten times as much, so 56,000. Many of those varietal names have a very loose definition, or no definition at all.

Cheers, Herwig

ACAM & MKA (Belgium)

29th Jul 2020 15:05 UTCRolf Luetcke Expert

Angel,

About a year ago, the Saint David area page had a couple of the "desert roses" labeled "selenite" and I contacted the people who had them listed as selenite and they fixed them but that was a usage I had seen not too long ago. I have always called them just gypsum myself.

29th Jul 2020 10:50 UTCTõnu Pani

Also, let me remind, that sources, based on the Cyrillic script, use name selenite mainly for gypsum fibrous form - satin spar.

29th Jul 2020 15:09 UTCRolf Luetcke Expert

Herwig had mentioned the sand included gypsum so thought I post one from the San Pedro valley in Arizona. These "sand roses" come from the East side of the valley, the others one above from the West side of the basin. When these sand roses have a broken blade, one can then see the inner gypsum but not on the complete rose.

29th Jul 2020 16:33 UTCRolf Luetcke Expert

Here is an interesting question to anyone who may know. We find a lot of the gypsum here in these big crystal blades but none are clear, they are all this light brown color although you hold them up you can see right through them. I just call them gypsum but I wonder just how wide the variety name selenite goes? I have not called them selenite but often people who see them do and I wonder if it is right to use selenite on these? I have not but would I be wrong to tell people these are not selenite?

29th Jul 2020 16:47 UTCDon Saathoff Expert

For me personally, selenite refers only to well crystallized, clear and colorless gypsum. But I may be alone....and I still label it gypsum.

Don

18th Aug 2020 10:01 UTCHerwig Pelckmans

Hi Rolf,

Did not see your question till now.

Wallerius is the one who coined the name "selenites" in 1747, for clear, colorless gypsum. So it does not have to be in nice crystals to call it that way, but it should be clear and colorless if you want to stick to the description Wallerius gave it.

See https://www.mindat.org/min-5527.html

HTH,

Cheers, Herwig

ACAM & MKA (Belgium)

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