Dear David,

It is not very clear from a single picture, but it seems to me that this crystal is tetragonal, so it cannot be quartz. Theoretically, in addition to the various aluminosilicates α-cristobalite is possible.

In my opinion, you need some additional information before making any conclusions. Is this the only crystal of this type in your sample? If so, is it really important? If not, do you have XRD data? What is Si:Al ratio from EDX? Did you try to make EDX mapping instead of the integral data? Is Al distributed homogeneously in the crystal?

Hi David, In my opinion, It is better you run XRD analysis and also TEM if it is possible. For even one data of SEM-EDX or EDS, I think you have to measure some areas (looks a Figure, about 5 marked area) but I suggest to rerun SEM-EDX analysis to gain more data , and  calculate the average in order to know the ratio of those components (Al, Si, and O: % atom ).

Dear David,

It seems like a aluminosilicates .It is important to observe the DRX results.

Dear colleagues,

maybe my previous answer was not shown. If I had no further information, I would suspect of being zircon. This silicate can even under harsh chemical conditions well preserve its crystal morphology. I can only support the suggestions for a more detailed mineralogical investigation.

Best regards

H.G.Dill

Dear all,

Thank you for the feedback and interesting discussion. Here is the EDX spectra of the particle. Maybe it can help.

Dear Alexander, in comparison with the massive presence of Fe in the RM, as also Ti and Al, the occurence of this type of crystal was minor. However, if finally I speculate that maybe is a zeolite - like aluminosilicate. In this case it is relevant. The XRD data show a predominance of Fe, Ti and Al oxides, but the presence of quartz and of course zeolite-type aluminosilicates. Unfortunately I did not carry out mapping of the particle, because I focused on Fe and Ti mapping.

Dear Rudy,

Thank you for your reply. Maybe I need more information to be sure

Dear Lúcia,

Thank you. I suspect of some zeolite-like aluminosilicate, but it does not exhibits the typical wool ball-like appearance of DSP. However, I saw a recent paper where similar zeolite crystals are described (but not in RM)

Dear David Rubinos: bauxite red muds have been treated, usually, with hot NaOH, therefore it is quite possible that the mineral you found is an alkali scapolite (marialite) formed by reaction of alkali, some chlorine present in the water with silica and aluminum oxy-hydroxide. Indeed, the crystal simmetry is markedly tetragonal, probably of P.G. 4/m 2/m 2/m, bipyramidal ditetragonal class, it shows large faces of a ditetragonal prism, cut by faces of a ditetragonal bipyramid, and a top tetragonal bipyramid, all proper of the scapolite series. Aluminosilicates Al2SiO5 are either orthorhombic (andalusite, sillimanite) or triclinic (kyanite), which is not the case; mullite is much too rare, and forms at very high temperatures. Tetragonal polymorphs of christobalite are more pseudo-cubic in aspect, and this crystal is bipyramidal. There are very few rock forming minerals of tetragonal symmetry, only zircon and vesubianite are frequent, but you don't have Zr, Ca, Mg or Fe as components, obviously. You should look for Na in the EDX, if the apparatus can analyze it, of course. DRX could be applied too but you will need a powder sample a little larger than a single 30 micron-sized crystal. With regards. (saludos) Sebastián. 

Dear Prof. Dill,

Vielen dank. Thank you. Yes, initially I suspected of ZrO2 too, because it is known that it occurs in this RM. However, the EDX spectra was confusing, cause it showed mainly the presence of Si and Al...then I suspected of quartz, that also occurs in RM, but the presence of Al was again confusing...I speculate that maybe is a zeolite-like aluminosilicate (which are known to occur in red mud) (similar appearance, but not exactly, to AFX zeolite) (please see image of this recent paper: http://pubs.acs.org/doi/abs/10.1021/acs.cgd.6b00365?journalCode=cgdefu...It would be plausible this (or it is unlikely)?

Thank you

Tschernichite tetragonal polytype, and gonnardite, are rare tetragonal zeolites. Point Group 422 could also generate a singony similar to 4/mmm, this could explain the presence of some Ca and Mg, and less Na as in tschermichite. Gonnardite is Na-rich and usually fibbrous, so it can be excluded. Saludos. Sebastián. 

Prof. Dill,

Just a correction, evidently I was talking of zircon, not ZrO2. Sorry by the mistake

Muchas gracias Sebastián!

Dear David,

looking at the intact and well crystalline shape of this mineral. I assume that it is a  resistate heavy mineral. The surface of this mineral is not clean and probably contain small contamination of the surrounding matrix. In my opinion, the presence of Na, Ca, Mg, Fe and Ti are probably derived from these impurities from the surrounding matrix, particularly that this is a rough surface. So, I assume it could be one of the aluminosilicates Al2SiO5 polymorphs (kyanite, analusite and silimmanite). these mineral are considered as resistate heavy minerals.

regards

Walid 

Reply to Walid Salama: Al2SiO5 polymorphs have neither such a bipyramidal habit nor this marked tetragonal symmetry (the 8 faces of the ditetragonal prism are too well developed, the other inclined faces could be 8 faces of the ditetragonal bipyramid). The only possible tetragonal resistates in the bauxite mud could be zircon, xenotime or monacite, but the EDX composition doesn't fit. By the way, David, I have some questions form you which might help to clarify this mystery: 1) Which kind of bauxite is the one being processed? It is a karstic one (like that of Jamaica or the Mediterranean region or a lateritic tropical soil (like Australian, African, Indian or South American bauxites, including the Venezuelan Los Pijiguaos deposit). I assume it could be the first option, since David is in Spain..., unless he is working in a tropical country somewhere... 2) Which is the possible source rock of this bauxite: granite, gneiss, nepheline syenite or foyaite, basalt, phonolite, rhyolite? This could help to identify the possible resistate in the bauxitic soil, and 3) How abundant is this mineral in the mud sample? If it is not a resistate, it could be a rare tetragonal zeolite, due to its high contents in Al, Si, Ca and Mg, which could form a low temperatures in such muds. The Na content is too low to be a marialitic scapolite, so a Ca zeolite is more probable. Another crazy idea: It could also be a new mineral species, who knows! Regards (saludos). Sebastián.   

Reply to Sebastián:

:) I don't think it is a new mineral...sure it has been described before! Thank you Sebastián for your assistance and helpful discussion. The red mud was sampled from ALCOA refinery in NW Spain, but to my knowledge they process bauxite from Guinea Conakry. This mineral is not so abundant...it appeared very scarcely, at least in my SEM observations, but it called my attention (in comparison with the rest of the red mud matrix...all oxides). Idk, maybe it is a zeolite, but it really looks like zircon! Definitely I need more information to confirm the EDX. Gracias!

Surely zircon is the best possibility, the tetragonal symmetry, the bipyramidal habit, and well developed, little corroded crystal faces, but your chemical data don't show any Zr but instead lots of Al, and some Ca, Mg, Fe and Na... Therefore you should check de EDX analysis for the presence of Zr-Hf, the Al, Ca, Mg, Fe and Na registered could be contaminants from the bauxite mud. So your bauxite comes from Guinea, therefore it is a typical tropical lateritic soil, probably developed on granitic or gneissic rocks of the NW African Shield, similarly to Venezuelan bauxite, where zircon resistate crystals are normally found. After all Guinea and the northern part of the Guiana Shield were welded together in Gondwana, before Pangea disruption, some 180 Ma ago. Regards. Sebastián. 

Is Topaz possible？