Chromite grains have been observed with exsolution patterns before, including the fascinating example of clinopyroxene and coesite from the Luobosa ophiolite (Yamamoto et al., 2009) and earlier than that the umixing of different spinels from metamorphosed chromite in the Red Lodge district, Montana (Loferski & Lipin, 1983). However, in this case the exsolved phase is no longer observed and in nearly every case there is a distinct chloritic halo. So, the question is (i) what was the exsolving phase? and, (ii) why is there currently only chlorite to be observed associated with these exsolved chromite grains?
Dear Christopher,
According to Ramdohr (The Ore Minerals and Their intergrowth) chromite may contain exsolution patterns of hematite, ilmenite, eskolaite and ulvite reported in the chromite. Hematite and ilmenite form platelets parallel (111) or networks that can account for 20% of the grains. Iron-rich chromites of the Bushfeld complex contain ilmenite lamellae which can have sub-lamellae of hematite.
Please find enclosed an image of ilmenite lamellae in chromite from Ramdohr representing very similar patterns like yours.
Best regards,
Guenter
Thanks Guenter. I agree that it has to be along the {111} plane and thank you for the Ramdohr reference.
There are two factors that still confuse me. (1) Where has the exsolved phase gone? There is no evidence for hematite, ilmentite, ulvospinel or eskolaite with any of the chromite grains and there are plenty of these grains to observe. The exsolution has occurred and the phase has just 'disappeared'. (2) If it hasn't disappeared then it has converted straight to chlorite to judge by the common association. That doesn't fit any of the four minerals mentioned by Ramdohr.
Dear Christopher,
Many thanks for your quick response.
Let me share some more ideas about your material. Chromite is very brittle. And the grains are easily broken under tectonic pressure. The cleavage of chromite appears very rarely. But the cracks are generally curved and irregular. In your case, the cracks are clearly oriented along the flat surfaces of the exsolution lamellae. The cracks furthermore show an increase in volume of the rock during alteration and the open fractures are filled with chlorite and or antigorite? This observation emphasizes that the exsolution phase was easily vulnerable to fluid phase and disappeared. The presence of chlorite and/or antigorite? strongly suggests metasomatic exchange.
What do you think?
Best regards,
Guenter
Dear Dr. Brough,
where did you take your samples from? Has the sample been taken from the area in the reaches of supergene or hypogene alteration? Basic and the more so ultrabasic rocks are very vulnerable to alteration processes of the two types. Even if chromite per se is rather stable and may appear as a heavy mineral in placer deposits its exsolution products are not always in the same position. Take ilmenite, which due to its bivalent Fe can easily convert into anatase and brookite. I have attached to show a chromite ore for a podiform chromite deposit in Turkey to give you an idea what a spade of minerals you can expect under certain conditions in a ore mineralization whose major minerals is from the mineralogical point rather stable.
I will wait for your answer rather than keep on speculating which way the elements have gone. I agree with the primary minerals found as exsolution products. This is all correct, but what comes in the aftermath? The answer can only be given based upon your knowledge of the geological setting.
Best regards
Harald G. Dill
Thanks Guenter and Harold,
Yes, I reckon the exsolution phase must be amenable to the fluid phase and easily transferable under metasomatic exchange. Actually, one next step might be to analyse the chlorite in detail - to determine whether it is kammererite, clinochlore, chamosite etc... and whether that gives any pointers). To expand a little on the tectonic setting. I have seen this texture in two separate settings both of which are in ophiolitic chromitites.
For the first setting (Turkey) this texture is abundantly present within the slightly more disseminated chromite lithologies but nevertheless present in both massive and disseminated chromitite. There is petrographic evidence for amphibolite facies metamorphism with the growth of other minerals such as cummingtonite and anthophyllite in the host rocks.
For the second setting (UK) this texture was only observed once, in one chromite pod. The ophiolite itself has not been regionally metamorphosed to amphibolite facies metamorphism but a recent paper argued that this chromitite locality in question (with the observed texture) had been to amphibolite facies metamorphism owing to close proximity to the basal thrust. They weren't interested in the texture (and I think they didn't observe it) but were deducing the metamorphic facies for other reasons. Nevertheless, it seems and interesting correlation between the two locations.
https://books.google.az/books?id=f5vPAwAAQBAJ&pg=PA165&lpg=PA165&dq=What+mineral+was+exsolving+from+this+chromite+grain+and+where+has+it+gone?&source=bl&ots=Ld6R34jnHh&sig=ACfU3U3ttTSHlQ4rEBbsRO6J6Ht8tRyzew&hl=ru&sa=X&ved=2ahUKEwjuwdOG7aLqAhWQZs0KHe9BAuQQ6AEwCnoECAkQAQ#v=onepage&q=What%20mineral%20was%20exsolving%20from%20this%20chromite%20grain%20and%20where%20has%20it%20gone%3F&f=false
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