Look for cobaltite.

Dear Ajay, It is certainly not cobaltite! Cobaltite is CoAsS, ideally containing about 45% arsenic. Our mineral does not have trace of As. It does not seem to be a very common mineral. 

There's a jaipurite CoS phase cited in Palache-Berman-Frondel (Dana's system...) but it's a long shot with the lapidary description you gave. Have you got some pictures? - both reflected light and BSE..

Dear Mr. Pal,

the major criterion to head for Co pentlandite is the chemical composition. The higher reflectance value also speaks in favour of this mineral. The anisotropy is for me second or third in the row when it comes to the mineral determination, as often anomalous features like that occur under the ore microscope. Look, there is also "anomalously anisotropic pyrite" which may come close to marcasite in its outward appearance. It is certainly not an answer satisfactory to all of us but we have to do some ranking of the various features and the chemical composition should be given priority. Reflectance and hardness (VHN values) I also used to place some emphasis upon. What is the mineral association like ? Maybe through the back door it works much better, if the key to the frontdoor is broken.

Also consult the paper by Yu. S. Borodaev et al. (2007) Pentlandite and Co-Enriched Pentlandite as CharacteristicMinerals of Modern Hydrothermal Sulfide Mounds Hosted by Serpentinized Ultramafic Rocks (Mid-Atlantic Ridge).

Maybe these hints help you constrain the unknown mineral.

Best regards

H.G.Dill

Dear Gavril! Thank you very much and sorry for my late response. Yes Jaipurite appears to be an option and is reported from the area where our sample comes from. Although this mineral (CoS) is not yet accepted by IMA, it has also been produced synthetically. This mineral has been briefly referred in Paul Ramdohr's book "The ore minerals and their intergrowths". However, we do not find any detailed description of the optical properties of Jaipurite and its compositional variability (range of concentrations of other elements viz, Fe and Ni). Our sample has significant Fe and Ni. Reported compositions of natural and synthetic jaipurite is almost stoichiometric pure CoS with Co:S ratio nearly 1. In our case, the metal:S ratio is consistently between 1.11-1.15 (10-15% deviation from stoichiometry) indicating non-stoichiometry and perhaps S deficiency. So we are still not very sure! I will appreciate if you can provide us with Palache-Berman-Frondel reference. Thanks again! 

Dear Mr Pal,

why don´t you describe where the mineral came from and disclose to us some more information as to the ore and gangue minerals associated with this dubious mineral. We have no idea about the precise morphology. Is it late- or early-stage ? What is the quality of the polished section like and how did you prepare the section ? Are some of the optical properties artificially created ? Why heading for the unknown and exotic, as the routine has not yet been done ? Answers to these open questions will certainly narrow down the field of speculation and prevent us from roaming around and at the very end will bring us faster to a solution.

Best regards

H.G.Dill

This is all I've got - not very much, but something to start with.

Dear Mr. Dill,

Thanks for your response and questions raised. I am sorry I could not get back earlier. I do not have any intention to hide anything. I thought the information I provided would be enough! I was mistaken and am sorry for that! Regarding your questions: The mineral comes from Khetri Cu deposit in Rajasthan, Western India. It occurs in a quartz-chlorite-amphibole vein in feldspathic quartzite (albite-quartz and accessory magnetite, ilmenite). The ore minerals include chalcopyrite+pyrrhotite+magnetite and rarely pyrite. The unknown mineral occurs only in pyrrhotite either as oriented blebs (exsolution?) and anhedral inclusion commonly of less than 50 micron size. The optical properties were studied in polished thin section which were prepared using a polishing machine. It was polished by standard method using silicon carbide powder, diamond paste and chromium oxide. Although in some samples, the host pyrrhotite did not take very good polish (pits present), this mineral always took a good polish. The optical properties that we see (as much as can be seen in optical microscope, we have NIKON 100 POL) are consistent in all grains and in all the sections. I think this answers the questions you have asked.

Regarding, the mineral being Co-pentlandite, yes our mineral is compositionally similar to that but as I had mentioned the anisotropism was the main concern. I understand and have seen (not in this deposit) anomalous anisotropism of pyrite. Just now I saw a paper (Francis et al., 1976, Orientation of exsolved pentlandite........American Mineralogist) that discuses anisotropism of pentlandite. I do not know whether Co-pentlandite can behave the same way. We would love to know if such anisotropism of Co-pentlandite has previously been reported. However, I must thank you for suggesting that pentlandite can show anisotropism. Thanks a lot indeed. Based on your suggestion, our subsequent literature survey and composition of the mineral, we now think that Co-pentlandite may be the best choice.

With best regards,

Dipak 

Dear Mr. Pal,

this is now sufficient. What about an oriented intergrowth of Co pentlandite with other minerals enriched in cobalt. Co pentlandite is a s.s.s. of Ni, Co and Fe and can alter into the bravoite s.s.s. (containing Co, Ni and Fe) under  supergene conditions or into minerals of the linnneite s.s.s. (Co, Ni, Fe). The latter shows a slight anistropy, in places, and used to form under late-stage hypogene conditions in several vein-type deposits. If the pyrrhotite  is absolutely fresh with no signs of supergene alteration, I would follow up the thiospinel pathway. The intimate intergrowth and the chemical variability may mask the true optical properties and give rise to an anomalous anisotropy. This is only a hint and I will not go any further because of substantial personal data. Maybe, that this "mineral" turns out to be such a decomposition product which under the EMP causes this special feature and gives as totals a Co pentlandite. It is certainly not the solution of your issue but an idea which you should check whether it is right or wrong. Even if we cannot exclude an unknown mineral, judging by the mineral assemblage the Co-bearing pentlandite and its decomposition products seem to be a plausible explanation.

Best regards

H.G.Dill

Dear Gavril, Thanks for the Geological Society of India Records. It was not in my collection and it is indeed a very important document for me. I much appreciate this! Regards! Dipak

Dear Mr. Dill,

Thanks for your suggestions! We have a project on this deposit and have just started working on the different issues of this deposit! I shared whatever I could. Hope you understand.

Thanks and regards!

Dipak

Welcome, good luck with the mineral!

Dipak

This paper maybe of relevance to your study

We found a hydrothermal intergrowth of pentlandite and pyrite adjacent to a chlorite–epidote halo, where pyrite replaces pentlandite.

Campos-Alvarez, N.O., Samson, I., Fryer, B.J., (2012). The roles of magmatic and hydrothermal processes in PGE mineralization, Ferguson Lake deposit, Nunavut, Canada. Mineralium Deposita 47 (4), 441-465

Hope it helps

Nelson

Thanks Nelson! I will certainly look into that paper.

Do you have any XRD data on it or do you have the possibillity to get? I have some samples from Gladhammar in Sweden with Linnaeite that also has some Fe inside (pyrite traces?), and the mineral is known to form series with the polydymite (Ni). I dont know if any of these could be relevant for you..     

Yes its the sulphide mineral lineiti in hydrothermal mineral deposit Co- is rich more than40 %.

Thanks Daniel! Firstly, the mineral is very small in size and volumetrically constitute very small fraction of the ore. Therefore, right now we do not have any scope for generating XRD data. Using micro-drill machine attached to the microscope, if  we can sample the mineral, then perhaps XRD can be done. However, we do not have scope for this. Yes, Linnaeite is a cobalt-rich phase but our sample is compositionally not Linnaeite as the metal/sulfur ratio is consistently between 1.11 and 1.15. In our case we can also rule out the possibility of pyrite traces (to explain high Fe content) as the compositions are very consistent in several grains. I believe inclusions of other minerals would result in more variable compositions of the target mineral. 

This mineral o yours may be a C-mackinawite. Similar mineral has been described fromTi-magnetite-sulphide deposits from Poland (Kucha, 1981; Chemie der Erde 40, 235-240. I attached a relevant publication. You have to do XRD, micoprobe, reflectivity, bireflectance etc. I reaslize it will be not easy beacuse mineral is small, but it is worthy to try because I thinh it is a new mineral. Cheers, Harry Kucha

Thanks Harry! We recognize that it is worth checking the mineral by XRD and reflectivity test. We have done EPMA. We also have Co-Mackinawite in these ores. To my understanding known Mackinawite does not contain so much of Co as this mineral in question contains. Please attach your paper.