Tilak, Howie in 1950's worked on the colour of charnockites and found that there are chlorite-like tiny vein lets in the quartz and feldspar giving them the greenish colour. However, the problems is whether it is really chlorite because under granulite-facies chlorite is not stable. As you are aware even the arrested formations of charnockite in Sri Lanka also have the greenish colour concurrently with hypersthene formation.

LRK sir, thanks for your participation. Presence of other minerals in quartz and feldspar as tiny inclusions may be one of the explanations. However, the question is why we have these inclusions only in minerals of chrnockites. These inclusions could also be incorporated into minerals in other rocks in the same terrain. If it is due to inclusions, as you said, it should not be chlorite since this mineral is unstable at high temperature. Also, how this color is developed in arrested charnocites if it is produced by inclusions. That’s a question. As you are aware, I have been working on chemical weathering of crystalline rocks including charnockites. In the case of chemical weathering of charnockites, disappearing of its greenish colour is identified as the first reaction. So, I am quite interested to know the root for greenish colour in charnockites.

Prof.Dr.C.Kasipathi, Andhra University, India.

Dear friend,

The issues of khondalite and charnockitre have been running for the last more than 150 years in international research with reference to Gondwana Research and Eastern Ghats Mobile Belt of India. As you know it perfectly well, it has been the mineralogical combination of quartz, feldaspar and hypersthene. Hypersthene can generally gives rise the greenish colour, which is not really conspicuous. In some of the regions, it may be a dominating feature, showing dominating green colour with those charnockites. But in many parts, say more than 95% of the entire charnockite-bearing EGMB of India, does not show dominating green colour.

Appearance of green later chlorite in charnockites is possible if they undergone retrograde metamorphism. In such case chlorite able to arise as hydratation product of hyperstene.

But spent two field seasons on charnockites of Northern Karelia I had never observe any distinct green colouration of these rocks. Quartz in them is glassy-grey or white and feldspars are pink.

Charnockite is metamorphic containing mainly quartz , feldpar and hypersthene in different quantities(weight%) depending upon the places (such as Nilgiri Hills, the Shevaroys , the Biligirirangan Hills and part of the Western Ghats(India) , Ceylon and Brazil). More so, Charnockite occurs in close association with other types of rocks.

Now the quartz is blue and opalescent, the feldpar has a milky shimmer like moonshine, the hypersthene has a bronzy metalloidal gleam(reported in the literature). The combination of these colors bands in different intensities can give to the greenish tinge.

As regards to the question of why therse minerals are included only , let me very humbly submit that all naturally occurring rocks can be divided into 7 types of crystals which are further divided into 14 types of Unit Cells( BRAVAIS LATTICES) having different porocities. Only these these contituents can fit well in their pore sizes(empty spaces/holes/ interstial sites).

Just a little note: Some of the charnokites at Varberg (Sweden) are greenish indeed, but some are as red as the granites nearby.

When light passes through the minerals certain wavelengths get absorbed and the remaining wavelengths in combination are perceived as colour of the mineral. This absorption of wavelength will depend upon 1. major element essential to mineral composition 2. presence of element as impurity in interstitial solution, for example presence of Cr in beryl will impart beryl green colour (emerald) while in corundum its presence in interstices will give it (corundum) red colour (ruby). Generally presence of chlorite in quartz gives quartz green colour. 3. Defects in crystal structure gives them different colours, for instance fluorite shows many colours. Quartz is smoky in colour due to omission (One of the four Oxygens attached to silicon get omitted, one can get transparent quartz artificially altered to smoky colour by exposing it to radiation). 4. Finely spaced boundaries like exsolution lamellae make feldspars look multicoloured due to iridescence and 5. finely dispersed admixture of impurities in host mineral, for example, electron transmission in Fe and Ti present as impurity in corundum gives corundum blue colour (sapphire).

Thanks Prof. Mahaveer for your detailed answer giving all causes for colors in minerals. As one of the responders mentioned earlier, it was thought that tiny inclusions of chlorite in quartz and feldspar are likely to be the cause for greenish color in rock charnockite. That is the same that you have mentioned in your answer. However, chrarnockite is a high grade rock which is metamorphosed under upper granulite grade metamorphism. Under this metamorphic condition, low-grade minerals like chlorite are not stable and can’t be incorporated in rocks even as inclusions. Therefore, the prevailed explanation for greenish color appearing in charnockite may not be accepted any longer. My question was there is any other explanation for the color of charnockite. Thanks in advance for your reply.

Yes, of course, nobody say that charnockite weren't formed under very high grade metamorphic conditions. But you should to note, that after high grade metamorphism often low grade (amphibolite, green-shist or even zeolite facies) metamorphism able to be superimposed on high grade metamorphic rocks. All charnockites are old age rocks, so they had a lot of time to be altered by later metamorphic and hydrothermal events under low PT conditions.

Of course this chlorite, mentioned above, hadn't formed during main stage of charnockite formation, but much more later during late metamorphic events.

Make a couple of thin sections of your green charnockite and you'll see chlorite (if it really is present here) scales within fissures in it very easy.

Colourless or pink K-feldspars able to become green due to incorporation of Tl+ or Pb+ ions in its structures instead of K+. But this process (amazonitization) take place in quite diffrent geochemical situation than we have in charnockites.

In alkaline rocks feldspars and nepheline may be coloured by exsolution of late bright green aegirine with composition close to its end member. But chharnockites aren't alkaline enough for this.

From other hand, superimposed tectonic events able to desintegrate orthopyroxene grains and make their colour more visible, more bright green. But this also should be well visible in thin sections.

And, of course, mineral colours not depends on their unit cells symmetry at all. For example tetragonal minerals may be any colour - colourles, red, orange, yellow, green, blue or black. Even worse, the same tetragonal can be different colours (colourless, orange, red, black - rutile, grey, green, orange - turkestanite and many other examples).

Usually colouration depends on local ion groups symmetry within structure, not on whole structure symmetry. For example bright raspberry-red colur of some eudialytes is caused position of iron within surrounding it 4 oxygen ions (very small fragment of whole unit cell of eudialyte) - the colouration appears when Fe ion located in the plane of these 4 oxygen ions (the same colouration center we have in structure of gillespite). When iron ion is out of the plane of oxygen ions and whole group of 5 atoms become pyramydal - this colouration disappears. It isn't exactly clear which conditions ruling position of iron in this structure fragment, but this fenomen has important indicator importance. In Khibiny apatite mines such bright raspberry-red eudialyte grow only within apatite ore bodies. The same pegmatite crosscutting apatite ore body has raspberry eudialyte http:// www.mindat.org/photo-317404.html within apatite ore, and brownish or usual red http://www.mindat.org/photo-581657.htmleudialyte outside of ore. http://www.mindat.org/photo-471577.html - within ore, http://www.mindat.org/photo-146704.html - outside of ore. But total symmetry of both eudialytes cells is the same.

Colour of the rock is cumulative colour of all the minerals taken together. The pyroxenes in charnockite are of green colour. As far as quartz is concerned please try to observe some quartz crystals, if you can scoop some of them from the rock, and try to see them under SEM. Let us see what do we find!!!!!!

Having worked on charnockites from southern Norway, I have seen a couple of examples of charnockites in freshly-dynamited road cuts that are grey. However, revisiting the outcrops a year later they have turned green. I wonder if the "chlorite" explanation for the colour is correct, but with the chlorite not being the result of retrograde metamorpism but of recent weathering and rehydration

Dear Craig, I am afraid, that 1 year isn't enough time for significant weathering and rehydratation of granitoid rock especially in climate of Norway, even Southern. Probably some blue-green algae had grow on the rock instead of chlorite within it.

Ortopyroxenes are stable minerals against of superficial weathering. Of course they aren't so resistant to weathering as calcian pyroxenes (diopside or augite) and gradually replacing by serpentine + limonite +/- opal. Chlorite can to replace hypersthene in hydrothermal conditions of green-shist facie.

You remind me one case of instantaneous oxidation of mineral contacting with water. When I was only student, we had field trip to Mikhailovskii quarry http://www.mindat.org/loc-194714.html. Jaspillites also are high-grade metamorphic rocks and in some layers they were green coloured by a "green mica" - http://www.mindat.org/photo-211642.html . I was ordered to collect samples enriched with this green phyllosilicate for further investigations by aspirants of our cathedra. I had try to do better, but it had occurs as always. I decided to concentrate this "green mica" by washing in pan and sedimentation of the phyllosilicate. I had collect couple kg of rich green jaspillite, crush them in powder and begun panning. Immediately after contact of dispersed "green mica" with water it had oxidized, and I obtained pan of cranberry juice. After drying of residue no relicts of green mineral weren't found, so it was completely decomposed during 1-2 hours. But here we have reverse transformation from green to red...:)

17 years later this "green mica" was described in the article Tsekhovskii, Y.G., Lavrushin, V.Y., Kuleshov, V.N., Shchepetova, E.V., Stukalova, I.E., Nosik, L.P., Prokof’ev, V.Y., and Areshin, A.V. (2002): Catagenetic Mineralization in the Platform Cover of the Mikhailov Deposit, Kursk Magnetic Anomaly. Lithology and Mineral Resources 37(3), 210-227.

Dear Craig and Pavel, thanks for the discussion and let me mention something that is fairly different to what both of you have written. My observation on charnockite weathering on the Sri Lankan terrain is contrary to what you have observed in Norway. The unweathered, intact, fresh charnockite in our terrain is usually green in appearance and that colour is immediately disappeared and the rock will look like as other gneissic rocks soon after the first reaction of chemical weathering. This can be clearly observed in “coresones”, which are the separated rounded blocks of bedrock. I am not a hard-core petrologist and my research interests are mostly biased towards earth surface processes as a surface geochemist. However, I am concerned about the characteristics of fresh bedrocks since I have been studding on how bedrock is converted into weathering products. So far, I have observed several thin sections of chranockite from very fresh bedrock to fragile weathering crusts and did not find any chlorite either as a weathering or a retrograded product. Also, I disagree with Pavel, who mentioned that opx is stable against surface weathering. The opx in chranockite is the first mineral that is undergone chemical weathering while attacking by reactive fluids. The paper appeared in Geochimica et Cosmochimica Acta (118 (2013) 202–230) widely deliberates weathering of charnockites under tropical setting. If I will again turn in to the initial question, I would suspect that greenish colour of quartz and feldspar visible in charnockite is likely caused by mineral opx (the reason is hitherto to known!) and not by chlorite.

Feldspars can accommodate a small substitution of Al by Fe3+ which can contribute to the red colour in feldspar. One wonders whether reduction of such Fe would not similarly contribute to a different colour. McIver J took "dark green" charnockite (unaltered) and placed it in an acid solution for a period of time after which the charnockite last its colour - re analysed the rock and concluded that Fe contents had been reduced. McIver, J.R. (1963) A contribution to the Precambrian geology of southern natal. PhD thesis. (Unpubl) Univ. of Witwatersrand. 203pps.

Interesting to explore. Current understanding as we all aware, is the veinlets of micro-scale in feldspars and quartz. But such (greening) features are absent in other granitic rocks of granulites facies as charnockitization is a process triggered by Co2 rich fluid migration at low water fugacity (such fluid infiltration is usually not seen in 'usual' granulites, thus greenish appearance is almost restricted to charnockites). In Kabbaldurga typical charnockite terrain, this has been explained as due to migration of volatiles low in H2O, and probably dominantly CO2. Dark colour (more often, variable greenish) of charnockites arises NOT ONLY from numerous tiny veins of chlorite BUT manganese-bearing calcite, particularly in feldspars (https://www.nature.com/articles/278511a0).

I just discovered this debate and was intrigued by your observation Craig, recalling that HCl treatment of Arendal charnockites caused them to turn grey (reported by the late Dennis Field in one of his papers). I think the green colour is related to deposits from a fluid along grain boundaries, being unrelated to the high grade assemblage. These rocks frequently form with beta-quartz, which upon cooling into alpha qz shrink a bit, making space for minor deposit, and I think some workers have shown that they are the cause of the ‘coloration malgachitique’. Perhaps it is possible to restore in some cases a sort of charnockite colour by chemical weathering where similar components are present? at least rational arguments should not overrule a good observation. In geology observation is always the first step!

Dear Craig, there is this oldish paper from the authoritative Howie (late too), who attibutes the color to thin graphite deposited along mineral grain boundaries. These would be beyond optical resolution, and won't show up in SEM either due to their elemental lightness combined with thinness, allegedly deposited from high-grade metamorphic fluid, allegedly rich in CO2. You should be able to locate the paper online.

BTW, thin cleavage flakes of graphite sometimes indeed look greenish in transmitted light.

Howie, RA (1967): Charnockites and their color. Journal of the Geological Society of India (8): 1-7.

A typical greenish luster characterizes the granulite gneisses of the western Grenville Province (Ontario) but is not related to the presence of chlorite flakes.