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Mineral that appears close to a conichalcite specimen
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nitana2000




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PostPosted: May 03, 2008 14:12    Post subject: Mineral that appears close to a conichalcite specimen  

Good afternoon. I have a piece of conichalcite bought in a collection that appeared in press. This piece does not have more cataloguing that its own name and the origin place, in this case is Mexico. I do not know any other data on its location. The piece measures about 5 cm. and has little density. The brown matrix type limonite undoes enough with facility. In one of the faces appears crystallized a mineral, that I do not know its name, and I would like to know that mineral can treat. I up some photography of the same, in which it is possible to be seen with a crystallization is transparent and with star form. The size of the same is between the 3 and 4 millimeters of diameter. Thank you very much for your attention, and sorry for my English because I don´t speak it so well. Thank you again.


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John S. White
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PostPosted: May 04, 2008 04:46    Post subject: Re: Mineral that appears close to a conichalcite specimen  

I am hoping that Peter Megaw will weigh in on this one, but I must say it makes educated guessing so much more effective when the photographs are as excellent as these are. My personal guess is that these are gypsum crystals, but they could also be aragonite, both of which are reported from Mapimi where this specimen probably came from. With a microscope it would be very easy to determine if the crystals are monoclinic, and therefore likely gypsum, or orthorhomic and therefore likely aragonite. An acid test could also be applied on a loose crystal. Aragonite will effervesce in dilute HCl, gypsum will not.
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nitana2000




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PostPosted: May 04, 2008 07:46    Post subject: Re: Mineral that appears close to a conichalcite specimen  

Hello, thank you very much for your attencion Mr John S. White. In the HCl test with a very small crystal, I don´t see effervesce. In the Spanish forum, Fabre think that the mineral could be "Creedita". I up a new photografy because I think that this can see a little best. Thank you very much for your answer, because I know that is very difficult say what is the with mineral with only photos. Thank you very much.


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KDF-TX




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PostPosted: May 04, 2008 09:33    Post subject: Re: Mineral that appears close to a conichalcite specimen  

I would guess Hemimorphite and from Mina Ojuela, Mapimí, Mun. de Mapimí, Durango, Mexico
I have many in my collection that appear exactly like that one.
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Pete Modreski
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PostPosted: May 04, 2008 09:49    Post subject: Re: Mineral that appears close to a conichalcite specimen  

Those are very good photos. When I saw the first ones I thought of hemimorphite, gypsum, and creedite; yes, aragonite could perhaps have been a possibility, too. Thought I'd wait to see what more informed people might offer any comments before I tried to.. I think they don't quite look like hemimorphite--a little too acicular, hemimorphite would be more bladed with more truncated terminations. I did think creedite was and is a good possibility, especially with the appearance in your last, very clear photo. I'd vote for that.

Pete Modreski
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lluis




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PostPosted: May 04, 2008 10:29    Post subject: Re: Mineral that appears close to a conichalcite specimen  

Good afternoon,
I guessed same as you, witout having a sample in my collection.
Today I have one and have seen another, from Mapimi (you know, a mineral fair, and flesh is weak...So...I simply could not resist)
Hemimorphite. First time in Tucson this year (or I have been said so...)

With best wishes

Lluís
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nitana2000




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PostPosted: May 04, 2008 10:59    Post subject: Re: Mineral that appears close to a conichalcite specimen  

Thank you very much for your answer, Lluis, Kdf-tx and Pete Modreski. So the mineral could be Hemimorphite or Creedite... Can I do something for know what is exactly?... The crystals are totally flat, like swords and totally transparents. Only one thing more, to KDF-TX. Are you sure that you have Hemimorphite and there are as this one?. Thank you very much.
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nitana2000




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PostPosted: May 04, 2008 11:03    Post subject: Re: Mineral that appears close to a conichalcite specimen  

One thing more: I have the piece in my collection about five years old. Thank you very much.
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lluis




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PostPosted: May 04, 2008 13:11    Post subject: Re: Mineral that appears close to a conichalcite specimen  

Good afternoon

Well, I could not see the specimen of KDF-TX, but mine is like yours (just bigger)

Creedite is a complex sulphate of calcium and aluminium.
Hemimorphite is a zinc silicate.

Apart from an XRF, an analysis could make it easier

Disclaimer: chemical reactives are dangerous: Should wear protection glasses, gloves, discard properly the rests and be very careful. If you are not trained for this, forget.

-Take a small piece.
Add to a test tube the sample.
Add sulphuric acid and heat
If it wets not homogeneously, then fluorhidric, and possibly creedite. (I mean, seems tears flowing, that means attack of the glass)

-Take a small piece.
Add caustic soda. Boild.
Filter.
Add some sodium polysulphide.
A white precipitate is zinc and then hemimorphite

Again, and being stubborn, as KDF-TX, it looks like hemimorphite

The pieces appeard in Tucson this year, but does not mean that were in the market before...

With best wishes

Lluís
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nitana2000




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PostPosted: May 04, 2008 14:23    Post subject: Re: Mineral that appears close to a conichalcite specimen  

Thank you very much Lluis, sorry but I only have HCl (18 %), and I think that this experience with chemical product are for specialists in laboratories, as you say is dangerous and I think that I don´t have too much mineral for this practice and without experience. When I answer what can I do, only I say for doing and say you. Thank you very much.
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nitana2000




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PostPosted: May 05, 2008 02:05    Post subject: Re: Mineral that appears close to a conichalcite specimen  

Good morning. As say Lluís and Fabres, the green mineral is Mimetite and not conichalcite as say I in the start of message. The colour of the ray is white. Thank you very much.
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Pete Modreski
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PostPosted: May 05, 2008 09:26    Post subject: Re: Mineral that appears close to a conichalcite specimen  

I was wondering about the correct identity of the green crust as conichalcite vs. mimetite. I presume that Lluis and Jordi wrote to you offline to comment on this being mimetite, as I do not seem to find anything in the messages posted to the Forum, making this comment. I'm not sure about the identity of this mineral, either!

Thinking about your mineral some more, and looking at photos posted on Mindat, I'm still uncertain about the identity, but I still lean toward creedite. I was just looking at the pictures in the original (1974) "Encyclopedia of Minerals" volume; it has two photographs of micro crystals of creedite from Santa Eulalia, and I think they both look very much like yours.

Now, I do have a suggestion as to what might readily determine what the mineral in your transparent crystal clusters really is. One of the traditional techniques of mineralogy, determining the refractive indices of the mineral using a petrographic microscope and calibrated r.i. oils, would easily distinguish the several different mineral possibilities. Because these minerals, especially creedite vs. hemimorphite, have greatly different refractive indices:
creedite, near 1.48
gypsum, near 1.52
hemimorphite, much higher, near 1.62
it would just be a few minutes work to examine one or two crystals in a grain mount on a microscope slide in the right index oil, to unambiguously determine which they were; it would not even require measuring the full optical properties (optic sign and precise refractive indices, which would be much more time-consuming and require more crystals).
I'm sure there are people in many countries who could do this kind of determination, and I realize you are in Spain and I am in the U.S.; but, nitana2000, if you wanted to go to the trouble of mailing me a letter with a few crystals enclosed, I could easily make this determination. You would just need to enclose a couple of loose crystals (perhaps some are loose or have already broken off the specimen), no matter how tiny they are, and you could just place them inside a little piece of folded-up paper within the envelope, and they would be sufficient to make this measurement. And then I would let you, and the rest of the Mineral Forum, know what the true identity is. So if you are sufficiently interested to do this, you are welcome to mail this to me here in Colorado:

Pete Modreski
P.O. Box 150995
Lakewood, CO 80215 USA

best regards, Pete
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nitana2000




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PostPosted: May 05, 2008 10:51    Post subject: Re: Mineral that appears close to a conichalcite specimen  

Excuse me, I´m Sorry, I write on spanish forum too, and in that forum, Fabre and Lluis say that the green mineral is not conichalcite as I say in the start and they say is Mimetite. The ray of green mineral is white. I think that you give me a very good posibility to know what mineral is... But I have a problem now, How I give the crystal? I up a new photo with Line 0,5 mm. for reference. The crystal have less of 2 mm of diameter!!!. Thank you very much. I´m very happy for your answer. Thank you again.


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KDF-TX




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PostPosted: May 05, 2008 12:27    Post subject: Re: Mineral that appears close to a conichalcite specimen  

I assumed the green was conichalcite too and threw out a wild guess. The mimetite I have from that location is a lighter green, finer (smaller) crystal structure and doesn't display that kind of breakage.
I'm not an expert, really just learning (as you can probably tell). Pete and others would know better.
I'm curious though and hope to hear if you have it tested. I suspect a lot of the specimens I have were mis-labled (or don't include the associated minerals).
That's a very gracious offer Pete. I could keep you busy... :)
Kevin
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Pete Modreski
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PostPosted: Jul 10, 2008 16:21    Post subject: Re: Mineral that appears close to a conichalcite specimen  

To Juan (nitana2000 on Jordi Fabre's Minerals Forum,

Following up on our discussion of these minerals from back in May, I just wanted to let you know that I received your letter a few days ago, containing the mineral fragments of the transparent mineral that you had asked about. I am going to try to do what I said I would be willing to do, examine them in refractive index oils, and see if I can tell definitively which of the likely minerals--gypsum, creedite, or hemimorphite--these are on your specimen, associated with the green mineral [presumed to be mimetite].

I will write back to this place in the Forum, with my results.

Best regards,
Pete Modreski
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Carles Curto




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PostPosted: Jul 11, 2008 01:05    Post subject: Re: Mineral that appears close to a conichalcite specimen  

I believe the crystals are too flat to be creedite.
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nitana2000




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PostPosted: Jul 12, 2008 05:27    Post subject: Re: Mineral that appears close to a conichalcite specimen  

Mr. Pete: I am very happy for all of you say... For me was very diifficult give a little minerals fragment, but with enought patience a give it... Thank you for your help, thank you very much.
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nitana2000




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PostPosted: Jul 12, 2008 05:41    Post subject: Re: Mineral that appears close to a conichalcite specimen  

I up a photo for all knows what I sent you... Best regards.


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Pete Modreski
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PostPosted: Jul 23, 2008 13:54    Post subject: Re: transparent mineral--a question answered  

Juan (nitana2000) and all,
I'll remind you again of our discussion of Juan's small transparent, bladed crystals, from Mexico (guessed locality, Mapimi or perhaps more likely, Santa Eulalia), and associated with a green mineral, probably mimetite (originally assumed to be conichalcite). Juan posted another picture of a cluster of these crystals on July 12, with a pencil lead for scale.

So, I've had a chance to examine the mineral under my petrographic microscope, in calibrated immersion oils as I'd offered to do, after Juan mailed me some tiny fragments. The answer is, they are indeed hemimorphite. Their refractive index matches what is expected for hemimorphite--the indices (the mineral is biaxial) are in the approximate range 1.62-1.63; the published refractive indices for hemimorphite are (alpha) 1.614, (beta) 1.617, (gamma) 1.636. What I observed is nowhere near the indices of gypsum, which are much lower (near 1.52) or creedite (lower still, near 1.48).

Thus, the several people who wrote and suggested hemimorphite as the most likely identity, were correct! I'm happy to have helped identify your mineral, Juan.

Sincerely, Pete Modreski
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lluis




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PostPosted: Jul 23, 2008 15:59    Post subject: Re: Mineral that appears close to a conichalcite specimen  

Good afternoon, Pete/List

If the german that sold the sample to Joan Viñals, who sold it to me, is not wrong, the locality is Mapimi.
Mina Ojuela to be exact.

With best wishes

Lluís

P.D.: a pity that minerals could not talk! They could explain so many histories...(as a grenat that was labeled as zircon and as a corundum ruby...despite its morphology :-( I have in my collection. I supose that I am missbehaving, but I love those little errors )
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