FMF - Friends of Minerals Forum, discussion and message board

[Andy Love]

Greetings Everyone

This is possibly a highly-unusual topic for this forum, in that it involves an unknown mineral that was not dug out of the ground on dry land! I'm attempting to discover something about how it came to be and this post is, in part, an appeal for guidance on how I might proceed from a mineralogical viewpoint. I hope you'll be able to bear with me while I (perhaps rather laboriously) describe the circumstances ...


I recently decommissioned one of my tropical freshwater aquariums. It was an acidic/softwater environment that had been running for seven years. Its floor consisted of a layer of sand, topped by a thinner layer of inert aquarium gravel. This assembly was laid over a heating cable (which provides a gentle vertical current that, in turn, extends the aerobic stratum). Apart from the attentions of a few burrowing snails, the substrate had been otherwise undisturbed throughout the life of the aquarium.

When I came to decommission it, I dug a trench in its substrate to see what changes, if any, had occurred. I was surprised to see that the sand layer was more-or-less pristine. However at the boundary between sand and gravel there were considerable amounts of: a) what was obviously iron pyrites ; and b) the mysterious black crystals that are the subject of this post. [In addition, as I later discovered when I dragged a magnet through some dried substrate, there was a small quantity of magnetic material. I have photographs of this material but won't include them here unless anyone deems them relevant].

I think that I understand the mechanism that formed the pyrites: bacterial reduction of sulphates produces hydrogen sulphide, some of which combines with heavy metals (in this case iron). But I was completely baffled by the black crystals: were they, too, formed biochemically ; or was this a purely chemical phenomenon? I'm on a quest to try to find out.

Although black-and-shiny in outward appearance, they are in fact composed of thin layers of nearly-transparent material.

Appended are a couple of pics of the crystals. I'm unable precisely to conform to the forum rules about photo attachments (because they formed in my living room ; and as I don't know what they are, I can't name them!) but I hope you'll allow me to present them.

Unable to make sense of them within the fishkeeping world, in desperation I threw myself on the mercy of a university's Department of Analytical Chemistry. To my great surprise and delight they invited me to bring them some samples, saying that they would attack them with a scanning electron microscope, plus X-ray analyser, which would reveal the chemical elements of which the samples are composed. So it was that I spent a fascinating morning at the lab and came away with some data.

Pic #3 is an SEM image of one of the crystals. Seven valid spectra were recorded from a selection of locations on several crystals [again I can supply images of them on request].

I loaded the reported mix of elements of each spectrum into the IMA identification tool. For six of them, as each element was added to the mix the number of possibilities dwindled eventually to zero. The only spectrum that threw up matches when I'd finished was spectrum #2. The elements present in that spectrum (with % by weight) were:

Oxygen 46.1
Silicon 27.7
Magnesium 12.0
Calcium 6.9
Aluminium 3.2
Iron 3.2
Sodium 0.8
Potassium 0.2

The IMA matches were:

Brinrobertsite
Corrensite
Stilpnomelane

A Google image search for those minerals failed to turn up anything comparable. An image search within FMF threw up one of stilpnomelane; it didn't look comparable - excepting that it had (what looks like) pyrites associated with it!

Removing the "IMA approved minerals only" tag from the IMA identification tool showed a fourth possibility : tibiscumite. I haven't found any images of that.

Elements revealed in other spectra, and which caused potential IMA matches to dwindle to zero as added were: Titanium ; Manganese ; and Chromium. Would it be 'safe' perhaps to regard these as contaminants (as it were) - meaning that the mineral is one of those four proposed above ... or is this genuinely something unknown?

Even if it's impossible to identify the 'species' of this mineral, is it possible to categorise it into a 'genus'? That might give me a helpful search term with which to take investigations forward into the mechanics of how it was formed.

Any and all comments you may have will be gratefully received. If there's further info that you'd like me to supply, I'll do my best.

Thanks for reading.

[DJandLIMON111]

Any way this could be a form of mica? Reading up on mica, I'd have to say that is what you have. The composition says "Basic potassium aluminum silicate, sometimes with some chromium or manganese replacing the aluminum" so with Manganese and Chromium I'd say you have some mica. Can you do a specific gravity test with one of them? Specific gravity can vary with mica from 2.7 - 3.0.

[kushmeja]

[DJandLIMON111]

Yes the hardness is 2.5, Mica is a group of different chemical structures i believe here is a link to the variations of it https://www.mindat.org/min-6728.html

[Dale Hallmark]

If it had been me, I would have taken a quick glance and concluded that it was escaped particles from my charcoal filter pack and threw it in the trash.

It is very interesting to me that you investigated the mineral composition. Very curious and I certainly don't have any answers.

Dale

[James Catmur]

A Zeolite from the filter?

[Dale Hallmark]

I think it possible.
Dale

[marco campos-venuti]

Nice puzzle!!
Starting with the assumption that X-rays don't lie, I would guess you have grown stilpnomelane crystals in your aquarium. Congratulations! The others two minerals are rare species of very similar composition that I would discard.
Stilpnomelane is very similar to the common biotite mica that is a metamorphic or magmatic high temperature mineral.
Stilpnomelane is a low metamorphic mineral, but can be also originated by chemical precipitation in the proper conditions.
Stilpnomelane is only famous to be abundant in Banded Iron Formations (BIF). If you don't know, just look at wikipedia, they are one of the most amazing rocks on earth.
The primary mineral assemblages of BIF is composed by: magnetite/hematite, jasper/silica, siderite/ankerite, stilpnomelane and locally pyrite. All of these are the result of chemical precipitation under anoxic conditions. There are suspect but no proofs of iron bacteria activity associated. The problem is that BIF are very old and usually deeply altered and metamorphosed, so it is quite difficult to assume that a mineral is primary. If in BIF it is not clear if stilpnomelane is primary, in your aquarium it is for sure.

[alfredo]

Remember that SEM/EDS is not very accurate for light elements like oxygen, magnesium and sodium; it just gives you a rough idea of the proportions. Furthermore, many elements substitute for each other in minerals, without being listed as essential elements in the formula, so, if you are listing the Na, K, etc in your search criteria, you are spuriously limiting the search results. Ca can substitute for Mg; Fe can substitute for both Mg and Al.... etc.

So the only elements you really know to be essential here are the Ca, Mg, Si and O.... which is a LOT of species! It could be augite sand (definitely not formed in situ), stained or coated with black organic material in the anaerobic depths of the tank. You should really send off a tiny pinch of those grains for PXRD (powder x-ray diffraction).

Here's another experiment you could try at home: Take a few grains of the sand in a stainless steel spoon (hold the spoon with a wooden clothesline clip or something like that) and roast them red hot for a few minutes in the kitchen gas flame or a blowtorch. See what they look like after all the organic matter is burned off.

[Pete Modreski]

Andy, one certainly can't resist reading and responding to a post titled "Mysterious black crystals..."!

My first comment would be, that those elements that showed up in the analysis---O,Si,Mg,Ca,Al,Fe,Na,K--are all very common elements that one would find in almost any "common rock" that one examined. So I wouldn't put much stock in any minerals that happened to match that combination of elements; you could probably get almost the same result by analyzing any pinch of ground rock or sediment or soil. In fact, I wouldn't "put any stock in it at all".

My other first thought, seeing the pictures, was whether these could be particles of charcoal, that you might have used in the aquarium filter--did it include activated charcoal? That's what they look like--very glossy and black. The XRF analyzer may be picking up all the various trace mineral components in the material, even if the bulk of each grain is mostly carbon, which is too light an element for most such X-ray systems to detect.

My other first thought was of it being some iron oxide (magnetite or hematite), but if so it really would have shown iron as the major constituent, which it didn't; and they wouldn't be at all transparent. Another though is obsidian, but I can't imagine that you would have introduced particles of obsidian in the sand or the filtration system. Though, if you were using some purchased, clean, "aquarium gravel", who knows what odd kinds of natural or artificial granular material, might be included in it?

Do you have any observations as to whether these black grains are quite light in density (as charcoal would be), or "comparable to the rest of the quartz sand", or particularly heavy (as iron oxide would be)? An easy way to tell is just how they settle out of water, compared to the rest of the sand--or as they would behave when sand is swirled around in a gold pan, for example.

Good luck with your mystery!

[Andy Love]

Thank you all muchly for your interest and replies thus far. I have been beavering away in reflection of your suggestions and guidance and, as a result, have more to add.

I will do so in a sequence of posts, if I may, such that my text appears more closely associated with my (rather poor!) photographs. Forgive me if I don't mention namecheck everyone who has replied as I do proceed.

Firstly, may I deflect the suggestions (from Dale, James & Pete) that the crystals may have something to do with the presence of zeolite and/or activated carbon (etc.). I don't use those materials in my aquariums. There's wood and leaf-litter, for example, but no 'industrial' products apart from some foodstuffs and the shop-bought inert gravel.

In which connection, my first piccie shows the black crystals mixed with the 1-2mm gravel : in case anyone wants to follow it up, it's called 'Crystal Quartz' and is made by a German company named 'Dennerle'. I have increased the proportion of crystals for the purposes of clarity.

My impression was (and is!) that the crystals essentially grew in the spaces between the grains of gravel.

[Andy Love]

Actually (DJ and Kushmeja) mica was my very first thought when I encountered these crystals. In 1970 I had an all-too-brief brush with a Geology degree (it was going relatively OK until X-Ray Crystallography took an intense dislike to me!) and, somehow, the memory of mica resurfaced.

I remembered its layers and how easy it was (usually) to disassemble them. So I took a scalpel to several of my crystals in order to try to do that : it was impossible. The first pic in this post shows the result of one attempt.

Applying a teaspoon-handle and pressing down with more-or-less all of my weight didn't work either. So I loaded some crystals into the jaws of a pair of pliers : that worked! Unfortunately, though, I didn't get nice sheets of material ; pic #2 shows the result.

I took some of the fragments and squinted at them under a microscope, as shown in the next two piccies. One of them in particular (I think it's the second one) may demonstrate the layered structure.

My conclusion was that the material probably isn't mica. Does that seem reasonable?

[Pete Modreski]

How about my question about the density of the material--does it seem more or less the same weight as the gravel particles? Sink just as fast when dropped into water?

[Andy Love]

Alfredo suggested (thank you) that they could be grains of substrate coated with black organic material originating in the anaerobic depths of the aquarium...

The sand layer was around 7cms deep at its deepest point(s). You'd normally expect the conditions in its lowest stratum to be anoxic. However: a) the presence of the substrate heating cable mitigated against that (somewhere I have a piccie of a trench that I dug in the sand and it's very clean right down to the bottom) ; and b) the crystals were formed above the sand, where conditions were definitely not anaerobic.

You further suggested that I do a heat test. In fact I had done a 'flame test' as soon as I'd come across the crystals, to see if there'd be a colour that I might be able to match to an element. I didn't take a photograph, though - largely due to my realising that I needed more hands to do so than I currently possess.

However at your suggestion I repeated the test, this time getting myself much more organised with a vice ...

Pic # 1 shows the torture in progress (I maintained it at red hot for half-a-minute).
Pic #2 shows the crystal before its torture.
Pic #3 shows the crystal after the burn. My photographic skills aren't up to revealing the detail, but it looked the same afterwards as it had done beforehand.

[Andy Love]