Dear Nima,

there are several good reasons why crystal shape should NOT be used to draw any conclusions about the crystal system of a mineral!

First, cutting an assemblage of cubes which are randomly oriented in 3D will never yield only square sections. In fact, the probability of one exactly parallel to (100) is rather low

Second, there are many many different shapes cubic crystals can take: octahedron ((111) planes), rhomb dodecahedron ((110) planes), trapezohedron ((hhk) planes) to name just the most common ones, and of course any combination thereof. None of these is likely to yield exactly square sections (think of garnets!).

Thirdly, shapes might be distorted due to preferential growth in one direction.

The only reliable criterion to identify a cubic mineral is optical anisotropy (always black under crossed polars), preferably in transmitted light.

On another note: I think your mineral is probably a spinel, close(ish) to the magnetite endmember of the series.  If you want to know exactly, you will need to do electron microprobe measurements to get the exact composition. EDX is only semiquantitative. If your crystals are compositionally zoned (wind up the contrast in BSE to see this), then you might even have several minerals in one crystal, going by composition alone. There are many minerals in the spinel group. If you want a good overview of spinel compositional variability get: Rock-forming Minerals: Non-silicates: oxides, hydroxides and sulphides. Volume 5A by J. F. W. Bowles, R. A. Howie, D. J. Vaughan, J. Zussman. It should have all the information you need.

That it is in an andesite makes it likely that it will be one of the spinels most commonly associated with igneous rocks, i.e. magnetite, hercynite or chromite. From your data it probably lies somewhere between magnetite and a point on the chromite/hercynite joint. Since webmineral only lists endmember compositions, it is no surprise you couldn't find anything there.

Hope this helped.

Regards,

Max

Hello Nima, may this be a spinel, something like hercynite with a high Cr content?

Hi Nima, I am not sure, but maybe it is something similar to aluminian chromite-like phase?

Have you looked a the mineral through a petrographic microscope? If it is isotropic and has very high positive relief like a garnet, it may support the spinel hypothesis. Other optical properties may help you decide too.

Hello Vincent,

Many thanks for your answer. I, as well, thought that it should be a chromium bearing mineral.

Your guess seems to be correct (Bravo!).

Anyhow, two issues remain!

Firstly, this mineral is seemingly not cubic, in contrary to hercynite (please see the attached picture).

Secondly, hercynite usually forms at high-grade metamorphic environments, while the host rocks of this mineral show only low-grade metamorphism (nevertheless, hercynite could also occur in mafic igneous rocks!).

Thank you again!

Hi Krzysztof,

Thank you for your answer. Yes, it could be, but I am looking for a name for the mineral.

Thank you Dr. M. A. Tyra,

The mineral is opaque under a petrographic microscope and is optically isotropic in some planes under incident light (although the mineral form as shown on the picture is seemingly not cubic). The relief is rather high.

Dear Nima,

there are several good reasons why crystal shape should NOT be used to draw any conclusions about the crystal system of a mineral!

First, cutting an assemblage of cubes which are randomly oriented in 3D will never yield only square sections. In fact, the probability of one exactly parallel to (100) is rather low

Second, there are many many different shapes cubic crystals can take: octahedron ((111) planes), rhomb dodecahedron ((110) planes), trapezohedron ((hhk) planes) to name just the most common ones, and of course any combination thereof. None of these is likely to yield exactly square sections (think of garnets!).

Thirdly, shapes might be distorted due to preferential growth in one direction.

The only reliable criterion to identify a cubic mineral is optical anisotropy (always black under crossed polars), preferably in transmitted light.

On another note: I think your mineral is probably a spinel, close(ish) to the magnetite endmember of the series.  If you want to know exactly, you will need to do electron microprobe measurements to get the exact composition. EDX is only semiquantitative. If your crystals are compositionally zoned (wind up the contrast in BSE to see this), then you might even have several minerals in one crystal, going by composition alone. There are many minerals in the spinel group. If you want a good overview of spinel compositional variability get: Rock-forming Minerals: Non-silicates: oxides, hydroxides and sulphides. Volume 5A by J. F. W. Bowles, R. A. Howie, D. J. Vaughan, J. Zussman. It should have all the information you need.

That it is in an andesite makes it likely that it will be one of the spinels most commonly associated with igneous rocks, i.e. magnetite, hercynite or chromite. From your data it probably lies somewhere between magnetite and a point on the chromite/hercynite joint. Since webmineral only lists endmember compositions, it is no surprise you couldn't find anything there.

Hope this helped.

Regards,

Max

it does look a lot like chrome-spinel. the Si and Ca picked up by the EDS are probably an common analytical artifact (secondary fluorescence) and don't really belong in that mineral. 

I don't think many people use mineral names like hercynite, chromite, spinel etc. in a very sensu stricto kind of way any more. 

Spinel for sure (magnetite rich); as Eric says Si and Ca are a bit odd and possibly due to contamination/fluorescence, but may also be present in the spinel lattice - as for Zn, Mg, Ti are typical for spinel.

Dear Max,

Many thanks for your interesting and comprehensive answer.

Yes, I agree! I guess that the mineral, as Vincent has suggested, is something like a hercynite. Probably a "chromian-zincian hercynite".

I have already found some articles in this regard, including the article from Louis R. Bernier (VANADIFEROUS ZINCIAN_CHROMIAN BASALT.HOSTED ALTERATION HERCYNITEIN A METAMORPHOSED ZONE. ATIK LAKE, MANITOBA,Canadian mineralogist, vol. 28, pp. 37-50, 1990).

Best regards,

Nima

Dear Dr. Hellebrand,

Many thanks for your helpful answer.

With best regards,

Nima Nezafati

Many thanks Dr. Gavril Sabau for the helpful comment.

Recalculation of your analysa to formula (Fe",Zn,Mg)(Fe"',Al,Cr,Ti)2O4 and ploting them to triangular diagrams Mg-Fe"-Zn and Al-Fe"'-Cr would help you to determine exact mineral name of your phases. I suppose that your crystals are zoned and there are represented at least two different minerals. This is typical situation for spinelides, which normally are very complex.

On photos given below you can to zsee zonation in crystals of zincochromite connected with their enrichment by Zn from core to rims.

Indeed the two bigger grains appear concentrically but diffusely zoned from the BSE image (lighter cores = darker hue, and heavier rims = lighter hue).

Chromite is the good candidate.

Dear Dr. Pavel Kartashov, Many thanks for your very interesting answer and the pictures.

Results from MEV are semi-quantitative but look on the image and mineral chemical data indicate that the mineral would be a spinel. 

Marcus Remus

Thank you professor Remus!

I agree with Kartashov and Remus--you need to plot this on a ternary diagram to get the mineral name, but you need microprobe (quantitative) data to do it right.  Your grains are probably magnetite (are they magnetic in hand sample??) with Cr-Al-Zn substitution concentrated more in the rims (where brighter) than the cores. If you can find discrete grains within the spinel of Fe-Cr-Al, you may have hercynite (Fe-Al), gahnite (Zn-Al) and/or chromite (Cr-Fe) intergrown with the magnetite.  Is there anything unusual about this andesite other than these grains?  Is there any propylitic alteration (chlorite, epidote, calcite) overprinting the andesite?

Dear Jason,

Many thanks for your answer. Unfortunately I have no access to EPMA for the time being.

Concerning your questions, yes the andesite is an amigdaloidal andesite and has born a low grade metamorphism (prehnite, pompleite, chlorite facies). The grains are not big enough to be examined in hand speciemen. I did not recognize any zonation in the mineral (using SEM).

Anyhow, I think the grains should be a kind of "chromian-zincian hercynite or magnetite". But hercynite seems to be more likely.

Best regards

I am working now on exactly similar mineral. this minerals can be called Cr-magnetite belonging to Cr-spinel group of minerals. in my case, this minerals is a part of reworked heavy minerals that deposited in a restricted sedimentary basin and then altered during oxidation. importantly, this mineral is zoned and in the inner zone Cr is more than Fe and this zone is chromite as you go to the outermost zone the mineral changed to Cr-magnetite. this outer Cr-rich magnetite is replaced by Si, Al, Zn and some Ti during chemical weathering or may be hydrothermal alteration. so if you are sure that Si, Al, Ti and Zn are present in the EDX or even in EPMA so you can look for any source of alteration or your analysis is not accurate and these additional elements are derived from associated chlorite or any surrounding minerals.

This is definitely a Cr-bearing spinel, most probably hercynite, as many pointed out. I have seen many like that in Cenozoic andesites erupted over a continental marginal arc. An important point is the content of TiO2 as it is indicative of oxygen fugacity.

Most likely chrome-spinel. All spinels contain two differing cations, or at least two different valence states of the same cation, in the ratio 2:1. This gives the general crystallographic formula AB2O4where the tetrahedrally coordinated sites are labelled A, and octahedrally coordinated sites, B. In general, the spinel types commonly associated with ilmenite concentrates are dominated by compositions containing the cations Mg, Fe2+, Fe3+, Mn (minor), Zn (minor), Ti, Al and Cr. The major cations substituting into the A site are the divalent cations Mg, Fe, Mn and Zn whereas substitution within the B site involves the cations Al, Cr, Fe and Ti. Aluminium, iron and chromium are present as trivalent cations however, substitution of Ti4+ into the octahedral B site (as in the case of ulvöspinel - Fe2TiO4), relies on a coupled substitution 2B3+=Ti4++A2+ mechanism, relative to the general AB2O4 formula. This gives rise to a range of spinel solid solutions within the system (Fe2+,Mg)(Al,Cr,Fe3+)2O4-(Fe2+,Mg)2TiO4 meaning that most spinels fall within a compositional space enclosed by the six spinel end-member components Fe3O4, FeCr2O4, MgCr2O4, FeAl2O4, MgAl2O4, and MgFe2O4. 

Cromite

Most probable it seems that the mineral is hercynite

I agree with others. It's a Cr-spinel in the chromite-magnetite series. Although variation in the elements in your analyses are typical of the spinel solid-solution series, you can also get some contamination due to micro-inclusions in individual minerals analysed. 

Agreed with others. Cr bearing spinel.

Dear Nima,

it will be helpfull if you will give here two complete analyses - with maximal iron content and with maximal aluminium content. In such case, it will be possible to recalculate them and at last to understand, is this hercynite or chrom-aluminian magnetite.

I also think it is Cr-spinel

Chromite-Cr-Spinel does seem possible? What oxide phases are present? How much Cr is in the magnetite? What is the composition of the plagioclase feldspar? Do  you think these are a xenocryst residue or a primary phase?

Al-Cr-Spinel

yes...it looks like Aluminous Chrome Spinel with may slightly Cr-,Mg-rich darker core and Fe-,Ti-rich brighter rim very common in igneous rocks.

Get some related data from Barnes and Roeder 2001; Journal of Petrology - J PETROL 01/2001; 42(12):2279-2302. DOI: 10.1093/petrology/42.12.2279

Yes, my guess is that you have a Cr-Fe Spinel, with some Al (it is not unusual, as well as traces of others cations, Ti, Mg, as  you mentioned)

Dear Nima,

It is obvious that your mineral is a spinel.

G. Christofides

Dear Dr. Nezafati,

it is without any doubt a mineral of the spinel s.s.s. It is a Cr-bearing normal spinel of the hercynite group, no magnetite spinel which used to have small amounts of Ti in an environment like that. The mineral shows a very homogeneous outward appearrance so that exsolution or replacement textures can be ruled out. As far as the crystal morphology is concerned the octahedron plays a significant role among the various habits. Sections are not the most appropriate way for morphological studies.

Best regards

H.G. Dill

Further reading on spinel (with images):

DILL, H.G., DOHRMANN, R., KAUFHOLD, S. and WEBER, B. (2011) Clay mineralogy and chemistry of fine-grained sediments. Environment analysis around the K/P boundary at Sekarna / Kasserine Island, Tunisia.- Neues Jahrbuch für Mineralogie Abhandlungen, 188: 285-296.

DILL, H.G. (2007) A review of mineral resources in Malawi: with special reference to aluminium variation in mineral deposits.- Journal of African Earth Sciences, 47: 153-173.

Dear Professor Dr. Dill,

Many thanks for your convincing answer and the references.  I agree with you that the mineral is a Cr-bearing spinel (of hercynite type) without zoning.

With very best regards,

Nima

Nima,

Petrographically I cannot tell, but its composition as determined by you and your identification of the host rock as andesite, not diagnostic however, suggest that the mineral is chromite. Go to the following link to see photos of chromite near the bottom of the page:

http://www.minerals.net/mineral/chromite.aspx

I maintain my opinion; the mineral is a Cr-Fe Spinel.

I think this a spinel

In my opinion this is a Cr-Fe Spinel

best regards

I also tip for spinel.

best

   In my opinion it is Cr-Fe spinel. Check the material which was used to polish your specimen. It may contaminate polished section.

Regards,

AP

I am in agreement with those opinions: Fe-Cr spinel.

Best regards, ZCS