FMF - Friends of Minerals Forum, discussion and message board

[Gerhard Niklasch]

Or, Some of the things that have caught my interest.

`Taking three as the subject to reason about...' let's have some sliced Tourmaline for starters! This is among the smallest pieces I have, and I do not know where it's from, though considering which stall I bought it from at the Mineralientage, it may perhaps be from the Transbaikal region of Russia.

We can see that it started out as a translucent, more or less triangular Rubellite, and later acquired a more or less hexagonal dark opaque overcoat. Also, it must have had a little triangular pedion face at one point, surrounded by three pyramid faces whose edges have left thin traces.

The second shot is a stereo pair. If you're new to these: hold a fingertip half way between your eyes and the monitor and look at it. Notice how your left eye now sees the image on the right behind your finger, and the right eye sees the image on the left; allow both images to blend into one. Gently remove your finger and without changing your crossed gaze, allow your eyes to re-focus onto the screen. With a little practice this will come naturally and succeed quickly...
Can you see that the various inclusions aren't all in one plane, and that the thin line from the triangle to the bottom right corner starts in the foreground and descends into the rearground? Also, notice the reflections on internal fracture or defect surfaces on the right, which are inclined in the opposite direction. If you look very closely, you'll see that the triangle in the middle is at the rear, with a smaller darker region in front of it.

This stereo pair was taken through my stereo microscope, using eyepiece projection, with both front and back lighting (halogen bulbs) if memory serves.

[Gerhard Niklasch]

Few things are harder to photograph than a lustrous black tourmaline. A black tourmaline sees everything: the wooden furniture, the red curtains, the green plants, the yellow shirt and the blue jeans, and you can bet that at least one of its faces will reflect part of that rainbow into the camera lens...

Here's one of these black guys - indeed one of my first tourmalines. It's from Erongo Mountain in Namibia. This one has started growing a three-ray star of a different kind: still solid where the main pyramid faces meet each other, most of the volume between the edges is beginning to separate into a myriad of baby tourmalines.

All the babies still end almost in the same planes, so at first glance, it looks like a trigonal pyramid with pits. (In later growth stages, each baby would terminate at its own height.)

The bright reflections show that many of the babies on the two shadowed sides of the pyramid already have little faces parallel to the large brightly lit face at the top.

For more views (including details and stereo pairs), please see https://www.mindat.org/photo-225604.html .

[Carles Millan]

Hi Gerhard!

>Few things are harder to photograph than a lustrous black tourmaline.

Right! Shooting schorls is a real pain.

Be welcome to this forum section. I hope to see much more pieces of yours.

[Gerhard Niklasch]

Many thanks for your kind words, Carles. Well my collection is still small, and I take images and process them from time to time as I get round to it. Your collection is, of course, a steady source of inspiration (and a constant target of admiration!).

It's been sunny today with a fine web of thin wispy clouds gracing the blue sky... an ideal occasion for taking a break from Tourmaline, and showing some Celestine. At 900g to the gram, this is one of the largest and heaviest pieces I have (so far). The vendor's label merely said `Madagascar'. Similar material is known, specifically, from the Sakoany mine in Mahajanga province; this specimen may also have come from there.

Most of the crystals are milky-blueish translucent, but a fingernail-sized part at the tip of one, conveniently sticking out at the corner, turns clear and transparent at a sharp boundary, and sparkles madly in the sun.

[Tobi]

Hallo Gerhard,

you wrote that the label says Madagascar and that the specimen may come from the Sakoany Mine. You can be sure: This Celestine is definitely from this mine. I've never heard of any similar Celestines from Madagascar, so you can be assured that it is from the mine near Sakoany in the Majunga province.

Liebe Grüße

[Gerhard Niklasch]

But somehow I'm always drawn to three-fold symmetries...

I have no idea where my second Tourmaline slice hails from. Maybe I'll find out at the next Mineralientage (if the vendor still has another from this series). Madagascar might be a possibility - one among many.

In the first pair of images, notice how the wide green bands to the left of the central dark triangle become very narrow when viewed from a different angle: They are traces of former pyramid surfaces, and evidently we're looking from that side where the tip of this pyramid had once been.

A stereo pair is next, taken from the same side, but with the slice repositioned to have these bands at the bottom.

Third and fourth, under the microscope with both front and back lighting, some interior detail. We're now looking at the other side of the slice, straight up into the former terminating pyramid. The brown inclusions remind me of sea weed...

All of the lighter greenish-white area between the bands is really a stack of ultra thin green pyramid layers alternating with clear layers, like the growth rings of a tree - while the bands that were so obvious to the unaided eye came from much thicker layers. This can be seen in the final stereo micro pair: each growth layer has left a thin green line along the yellow fracture surface running left-right through the centre. (The three white patches on the upper surface, two at the top and one below said fracture, aren't inclusions as far as I can tell, but pits filled with residue of the polishing agent.)

[Gerhard Niklasch]

Thanks Tobias - that was also my impression after rummaging among the possibilities on mindat. Plus, an imprecise location on a label makes it likely that the specimen came from a prolific source, not from somewhere unusual that would have been worth mentioning.

Still, we just had a lively discussion on the mindat forum about the pitfalls of spurious specificity and the value of being very clear what's known and what's guesswork! :)

[Tobi]

Oh yes, i know about this problem :-/ A lot of specimens came to me during the years, and many of them had just a vague information about the locality. When i buy minerals, of course i always look after complete and correct localities. But often you inherit a collection (or you just get some specimens as a gift) from another collector who didn't care as well about the correct localities. That's especially annoying when the mineral is something very beautiful (and potentially precious) like a large Fluorite or Calcite specimen. I often had to sort specimens out just because the correct locality was missing. That hurts when the mineral had the quality to become a really nice display specimen :-/

[Gerhard Niklasch]

I'm not being religious about it - I sometimes buy things I like even if they come with incomplete information; and trying to find out more about their possible origins is often quite instructive...
Like this piece of Fluorite from a locality called `Mexico' which I picked up for 3 EUR some weeks ago. (I then went through all 31 Mexican states on mindat looking for Fluorite occurrences, and decided that the Tule mine was a probable candidate.)
Cubes, by the way, also have three-fold axes of symmetry :) as the truncated corner on the right illustrates. (The red-brown stuff above/behind it is an inclusion.)

[Ed Huskinson]

OK we can see the close-up of the nice purple fluorite. A view of the entire specimen, and another of the back-side to show the matrix, would help us with the locality determination.

Thanks GN.

Ed

[Gerhard Niklasch]

Sure Ed! ...but I had to take these pictures first. (The specimen just happened to have wandered into a late-night microscope session before it met a convenient sunny morning...)
Here's a composite of views from four sides, with the above stereo micrograph taken from a fifth (the top). There is no matrix. The bottom is just a fracture / former contact surface.
The zoning (violet in several layers along the outside, mostly clear interior) is more pronounced in person than the pictures have captured, since I used no back lighting here.

[Gerhard Niklasch]

Calcium carbonate is the prototypical material of biogenic sedimentary rocks.
Olivine, red-hot and molten, is believed to make up the bulk of our planet's mantle.
So... what is it that brings both together in one and the same specimen?
...Metamorphism!
In particular, high-temperature metamorphism of impure dolomitic limestones. (Many limestones contain significant amounts of quartz or silicate impurities -- anyone who, like me in younger years, has been exploring the deep cave systems of the Picos de Europa can attest to their extreme abrasiveness. But I digress...)
So here we have, from a classic locality, a group of Peridot crystals of fine colour and frosted faces, with a backpack of translucent Calcite rhombs. (Magnetite is also present in dark, less than mm-sized, vaguely octahedral xx.)
I'm not sure how the belt running from top left to bottom right across the show side (and top right to bottom left past the Calcite on the back) has come about. It might reflect some tabular feature in the former matrix neighbourhood, or, perhaps more likely, it might be the result of the group having grown in a widening vein, torn apart and rehealing in the middle.

[Gerhard Niklasch]

And a green apple from the southwestern corner of Mali... The Prehnite ball, with a ligher and a darker half separated by a sharp boundary, has grown around some kind of axial matrix support; a little crisscross group of Epidotes is attached to one side. (This association is common for the locality.)

[Gerhard Niklasch]

And while we're on the topic of green things...
This specimen unfortunately suffered some harsh treatment in transport, so much so that another plastic box in the same parcel was cracked. Astonishingly, the `grass' of tiny Malachite needles held up rather better than the naturally crumbly dolomitic matrix.

[Jordi Fabre]

Gerhard,

I'm not sure about the right locality of your Fluorite: https://www.mineral-forum.com/message-board/viewtopic.php?p=4803#4803 but I would like to suggest Mina San Martín, Sombrerete, Zatatecas Mexico. Again, I'm not really sure about it.

Hopefully Peter can help.

Jordi