FMF - Friends of Minerals Forum, discussion and message board

[Jordi Fabre]

OK Master 2.

Anyway, I have a doubt. In https://www.mineral-forum.com/message-board/viewtopic.php?p=9227#9227 you said:

[Peter Megaw]

It is possible since the entire mountain seems to be riddled with gypsum pockets...but the Cave of the Swords and sister cavern where most of the historic gypsum specimens came in quantity from were in the Maravilla and Lepanto workings. I used to buy crystals by the carload from the mine manager...they harvested them from the Cave of the Swords, sold them and used th proceeds to pay for the refreshmensts served in the miner's club.

Unless you have some reason to think whoever made the label that goes with this piece really knew which mine it came from, you would probably be safest to put "upper oxide levels" "Naica Mine" on the label, perhaps noting a pre-1955 harvest date.

The mine is almost equally well endowed with anhydrite crystals, they are everywhere... except they don't choke up the pumping system and sumps or line enormous caverns. There has been a fair amount of work done on the origin/source of the gypsum and anhydrite...Google "Naica" and "Geology" and you will be led to the April 2007 article in the scientific journal "Geology" on the subject. Although it does have some significant flaws regarding crystal growth, it shows the that both the gypsum and anhydrite are derived from evaporite beds that occur in the host rock section below the principal limestone units. Both the limestone and anhydrite/avaporite beds at that depth are thoroughly thermally metamorphosed to coarse-grained marble...if that term can be applied to the evaporites. The ore fuids passing through dissolved the "primary" anhydrite and it reprecipitated as anhydrite with sulfides in the orebodies or gypsum on sulfides depending on the temperature. Interestingly the "secondary" anhydrite crystals we see from the mine are sky-blue to grayish blue in color whereas the bedded anhydrite at depth is lilac in color.

[Jordi Fabre]

[Gerhard Niklasch]

[Pete Richards]

Very often modifying faces on fluorite are misidentified. The usual confusion, when modifying faces of a cube occur in groups of three, is whether the faces belong to a trisoctahedron or a trapezohedron. Since the faces meet at a corner of the cube, where an octahedron would be if it were present, folks often think these faces belong to a trisoctahedron - the thinking being "trisoctahedron" - "three-octahedron" - "three [faces instead of an ] octahedron [face]". In my experience, these modifying faces usually belong to a trapezohedron, {311} being the most common. The Naica Mine fluorites are more complex, and I think Herr Niklasch is right in his analysis. But as a general rule, if the modifying faces appear primarily to cut off the edges of the cube, they belong to a trisoctahedron. If they appear primarily to cut off the faces, they belong to a trapezohedron. In either case, they modify the corner, with three faces replacing one hypothetical faces of an octahedron!

Trapezohedra have the general Miller indices {hkk}, where h is greater than k, e.g. {211}. Trisoctahedra have the general indices {hhk}, where h is greater than k, e.g. {221}. Since all three axes are equivalent in the isometric system, {211} can equally validly be written as {121} or {112}, where the curly brackets mean that the Miller indices represent all faces of the form, not just one face. {211} and {112} are common choices in different regions of the world; I think {121} is rarely if ever used when all three choices are equivalent.

Attached are a couple of drawings.

[Gerhard Niklasch]

Many thanks for the illuminating diagrams!

Yeah, I had been struggling with the identification for quite a while, and had to throw away several wrongly annotated pictures before coming up with the above.
(Never trust a mathematician who embarks on actual calculation!)

Perhaps it's just me - I'm finding finding myself even more readily confused when these forms modify an octahedron rather than a cube...

Cheers, Gerhard

[Peter Megaw]

Pete...taking this one step further. Apologizing in advance for the picture...here's a close up of a trapezohedron on a Naica fluorite...modified by a face that cuts off its triple junction. There would be 6 of these in the form, negative octahedron or negative cube...or something else altogether?

[Peter Megaw]

A few more nica Naica fluorites!

[Peter Megaw]

A break from fluorite...a couple "seldom-seens"

[Gerhard Niklasch]

[John S. White]

An old rule that I remember to help distinguish between the two forms described by Pete is to note the orientation of the Y formed by the faces of the form in the corner of the cube. If the Y is upright, the form is the trapezohedron. If the Y is inverted, we are seeing the trisoctahedron.

[Peter Megaw]

So...to coin a mnenomic..."Y"es its a Trapezohedron

[javmex2]

Hi, hope you like this photograph, is a real beauty of Mineral Naica Chihuahua Mexico.

[Jordi Fabre]

One new wonderful thread to the Featured Columns of FMF! This time the topic is the Naica Mine (see above)

Please continue developing it, a great mine needs a great thread! ;-)

[Mark Ost]

Beautiful