Dear Aniruddha, secondary biotite (or phlogopite) is commonly more fine-grained and it occurs in small clusters or veinlets overgrowing the primary mineral assemblage. It is usually intergrown with secondary magnetite phases. In cases of strong biotite alteration that is relatively obvious in thin sections. However, where the biotite alteration has only a weak intensity, the secondary biotite (or phlogopite) flakes can occur as fine-grained disseminations and their overprinting nature can be more difficult to identify. In most porphyry Cu systems the characteristic (potassic) biotite-magnetite alteration commonly actually consists of secondary phlogopite(!), rather than biotite. You can identify it by its brownish colour in handspecimen. By contrast, secondary biotite looks pitch black and it is actually more rare in hydrothermal systems. However, it is well developed at the large gold deposit at Lihir Island in P.N.G. where epithermal gold mineralization post-dates an early-stage porphyry-style mineralization.

PS: I have learned that many years ago from Richard Sillitoe who has studied many mineral deposits. :-)

Please find enclosed a PDF showing two photomicrographs of primary and secondary biotite right next to each other in a thin section of a greenschist-facies metamorphic tonalite with primary magmatic, porphyric texture. This is a sample from the Rieserferner Mountains in northern Italy.

The secondary biotites are more fine-grained and irregularly grown compared to the primary euhedral biotite crystals, which are partially bent.

Best regards,

Guenter Grundmann

You can distinguish them by textural features or from their chemical compositions.

According to Nachit et al 2005. Bitote can be classified into three groups.

1. Primary biotite: are usually greenish, occurs as indepent crystals and show higher TiO2 contents.

2. Secondary biotite: can be either re-equilibated or neoformed. They are usaually brownish or pale brown due to low TiO2. They occur usualy as exoslution lamellae, intertistional along grain boundaries or as overgrowth on the primary biotite.

Please read Nachit et al. 1985 and 2005 on the classification of biotite for a detailed insight.

Guenter Grundmann Thank you for the pdf. The images indeed are helpful and euhedral nature is surely one of the convincing criteria to distinguish between primary and secondary type. But can you suggest any other petrographic criteria apart from the euhedral nature, that is characteristic of magmatic biotites?

Saleh Ibrahim Bute I have gone through Nachit et al., 2005, and this paper mainly deals in chemical criterias. While in deformed archean granitoids, these chemical criterias have some loopholes. So, I was stressing on textural evidences. Later the textural evidences can be supported by the Chemical parameters suggested by Nachit et al.

Dear Aniruddha,

Here is another clear criterion and textural evidence for distinguishing primary magmatic and secondary metamorphic biotites,

please see PDF with images attached.

The relationships between crystallization and deformation of a biotite aggregate in a biotite-gneiss show unambiguously that

the primary biotite generation is strongly foliated and weakly bent, and the young secondary biotite generation is diagonally newly grown, with no signs of deformation.

Best regards,

Guenter Grundmann

For Harald G. Dill to reply this question.

Regards

Ijaz

Please find enclosed a PDF showing two photomicrographs of primary biotite phenocrysts in a thin section of

a) Sample from Rieserferner Tonalite, northern Italy: biotite phenocryst showing kink bands, plastic deformation lamellae which developed during magmatic intrusion.

b) Sample from the Drachenfels (Dragon Rock), Germany: biotite phenocryst in trachyte matrix showing fluidal texture. The biotite shows a primary magmatic fringe of tiny magnetite grains on its edge.

I hope this helps you further.

Best regards,

Guenter Grundmann

Dear Mr. Mitra,

Your question is rather unspecific as to the processes and the parent material (it can be volcanic or intrusive) and the secondary biotite can be of metamorphic origin or may still form part of the magmatic process (late to postmagmatic).

In principal, your question is a matter of compositional (1) and textural/structural (2) changes.

1. You can use micrographs and try and determine which mineral is replaced and which the newly formed one is. To do this successfully needs a lot of experience in microscopy and in each field of mineral alteration you are engaged in.

A way which in my opinion may achieve better results than anything else is a careful chemical analysis of the biotite as to its FeO, MgO and MnO contents in the lattice. Biotite like many other Fe-Mg-Mn silicates tends to increase the MgO/FeO ratio with rising metamorphic grade. Mn mostly is at lowest level of alteration. There are also O isotope studies or Mossbauer studies making use of the valence state of Fe. But before embarking on this journey I would try and make use of EMPA.

2. As far as the dynamo metamorphism and structural geology is concerned some remarks have already been given by Dr. Grundmann. Carefully analyze the micro shear planes and find out which biotite is correlated with it. In this case there are many textbooks on structural geology available showing fine photographs with different types of foliation and an all-embracing explanation in the captions.

I express a stark warning to carry out a simple image comparison. You have to “enter” into and “live” in your petrological system under study.

I wish you much success.

H.G.Dill

An image comparison, especially regarding rock microstructure, is anything but simple. It is the most effective way to make a complicated issue visible. A picture says more than a thousand words could ever do. Therefore, it must be strongly warned to refer simply to any textbooks, without mentioning specific references. Even in the world-class book by Ron H. Vernon (2004) "A practical guide to Rock Microstructure", Cambridge University Press, the distinction between primary and secondary biotites is not specifically addressed. Therefore, the contempt for petrographic comparisons is extremely unfair. It is even worse to advise a little experienced that he should first of all "live" and "enter" his "petrological system under study".

Best regards,

Guenter Grundmann

no comment

Hello,

You can also use the chemical composition of biotite to further investigate the origin of your grains. There are two studies using biotite electron microprobe data to do so. Miyano and Miyano (1982) and Nachit et al. (2005). I used the second method after petrographic studies and it worked quite good. It does need calibration for Li-bearing mica though.

Miyano, T., and Miyano, S., 1982, Ferri-annite from the Dales Gorge Member iron-formations, Wittenoom area, Western Australia: American Mineralogist, v. 67, p. 1179–1194.

Nachit, H., Ibhi, A., Abia, E. H., and Ohoud, M. B., 2005, Discrimination between primary magmatic biotites, reequilibrated biotites and neoformed biotites: Comptes Rendus Geoscience, v. 337, p. 1415–1420.

Regards,

Zeinab

Within a porphyry copper system, the secondary biotite forms as an envelope around a Cu-pyrite or Cu-Mo vein or veinlet whereas the primary biotite is likely altered within the alteration zone and fresh outside that zone.

Dear Aniruddha,

Your question requires a little more clarification about what type of rock you are working on. If you are working on a metamorphosed granite/granite gneiss and you are trying to distinguish primary (igneous) biotite and secondary (metamorphic) biotite, then please check the petrographic relations. A metamorphic biotite will have cross-cutting relation with pre-existing minerals (maybe older biotite, K-fs, muscovite etc). Same is applicable for biotite bearing metasedimentary rocks. If this metamorphic event is related to a deformation event, then all the primary biotites will be affected by the deformation event (folded/kinked/anhedral) and secondary biotites, which are more euhedral and less deformed than the primary ones, may define a fabric with growth perpendicular to maximum stress direction.

And, @Dr. Grundmann, thanks for sharing the beautiful photomicrographs.

Trisrota Chaudhuri

Thanks for your response. Your explanation is lucid and sums up what Guenter Grundmann explained and showed in his first photograph.

So, if I see some well-shaped biotite oriented along a penetrative foliation, can I assume them to be magmatic? In my case biotites are oriented along the foliation and show crosscutting relation with plag and other early formed minerals. But, biotites are not folded/kinked.

P.S. I should have mentioned the rock type I am working on. Sorry for that.

Zeinab Azadbakht I am aware with the criteria provided by Nachit et al., 2015, but as far as I know, chemical criteria are often associated with loopholes especially in case of Archean granitoids. So, I think one need to validate the magmatic character by petrographic criteria first then that can further be validated using chemical means.

Harald G. Dill I sincerely thank Prof. Gill for his input.

Dear Aniruddha,

If your biotites are euhedral, not folded/kinked, oriented.along a penetrative fabric and replace pre existing minerals, then greater chance is that these are secondary/metamorphic.

Trisrota Chaudhuri No, I dont see any biotite replacing any preexisting mineral. HOwever, they are oriented parallel to a foliation. But, as Prof. Grundmann's photomicrograph shows, the secondary one grew at a high angle to the foliation. I dont see any such thing in my slides.

I might have wrongly stated cross-cutting relationship in earlier reply. Here biotites are partly/fully enclosed by plagioclase crystals.

Based on petrographic study , secondary biotites are euhedral, macroscopically foliated and chemically contain higher Fe(O)t+MnO compared to 10*TiO2 and MgO.

Dear Aniruddha,

I have some experiments on distinguishing primary versus secondary biotites in Iranian porphyry copper deposits like Sarcheshmeh (PCD). In hand specimen, secondary biotite is so fine grained that you can only see the brown matrix in the rock and their flakes can not be seen with eye. The secondary biotites are like brown powder. But primary biotite exists as larger grains, sometimes as euhedral ones in the matrix.

Although their geochemical signatures are totally different as described by others, but it needs instruments like EPMA.

Kind regards,

Mohammad

EPMA data can easily separate two types of biotites. But in hand specimen, these types can be distinguished as explained by Mohammad.

Dear Mohammad, in rare cases, the secondary biotite can be very coarse-grained. For example, at Skouries (Greece) and Grasberg (Indonesia) the geologists have collected samples with secondary biotite flakes of up to 1cm. At Grasberg, they are called "pseudo-pegmatites".

In my opinion, it is possible to distinguish when examining the sample under the microscope, which is the presence of voids in the granule indicating the igneous Rx origin of the biotite mineral, that is, it is derived from volcanic igneous rocks and these gaps are the result of gas escapes

Dear Aniruddha Mitra

Regarding the chemical composition, comparing primary and secondary biotite from the same sample, it usually occurs that the latter is richer in Si and poorer in Ti. The loss of Ti can form rutile needles. In the specific cases of potassium alteration of porphyry copper, an increase of Mg in the secondary biotite frequently occurs.

Aniruddha Mitra It would have help a lot if you attached the photomicrograph of the case at hand. In my opinion, there may not be a rule of the thumb. If it occur to have such varieties, one will be able to resolve the puzzle with a polarized light microscope.