Dear Nathalia,

Based on the provided photo I can not be sure about the correct mineralogy. But, if you are sure that your mineral is uniaxial (+), it could be Quartz. As you know, Feldspars are Biaxial. In case of different appearance of some quartz grains, maybe you are dealing with two generations of Quartz and please have a more accurate look at the grain relations and boundaries in order to distinguishing the different types of Quartz occurrences. For example, in highlighted part of your image, below, you will find the quartz crystals packed together, looking like being mineralized as Quartz Veins.

Kind Regards,

Masoud Ovissi

Dear Nathalia,

The following facts indicate that the large grain in the SW is an albite-rich plagioclase:

The thin section shows a biotite-muscovite-plagioclase schist with crenulation cleavage.

The portion marked with yellow stripes is a quartz aggregate with weakly interfingering grain boundaries and undulous distinction.

The large grain in the southwest contains mainly inclusions of quartz and clinozoisite, outlining an old foliated internal structure.

The metamorphic grade can be classified as greenschist facies.

I doubt, that this mineral in the SW ist uniaxial.

Best regards

Guenter

Dear Guenter,

You are right. Based on the appearance and optical characteristics the large grain in SW should be feldspar. Checking the optical properties again, maybe, will resulted in Biaxial instead of Uniaxial.

Kind Regards,

Masoud Ovissi 

It looks like orthoclase. So you need to check the optical properties and its sign again, I doubt that to define as uniaxial. You can check again for more accurate. If quartz, it will give  a low relief and undulated extinction due to suffer high pressure under metamorphism condition. Thanks 

nepheline must be uniaxic -

and feldspars must be biaxics,   also the metamorphic minerals   should be biaxics,

treat an ancillary technic as a microprobe, raman,  xrd,    or treat as coleagues sugested,  to probe again the optical properties.

Dear Nathalia: I live in the Cordillera de la Costa, in northern Venezuela, where my colleagues and I have been studying Mesozoic low grade pelitic schists, in greenschist facies, during decades, very similar to the one you show. The mineral with lots of inclusions is poikiloblastic untwinned ALBITE. Its interference color is variable from 1st order gray to light yellow due to different optical orientation of the grains, of course. Actually since the parallel inclusions are oriented almost orthogonally to the foliation, the texture of the albite is helicitic. The optical determination in such metamorphic minerals, in deformed rocks like the one you show, is not easy to do. Albite has a large 2V angle, so the isogyres are almost straight and might be confused with uniaxial isogyres, but even been quite straight, the Albite isogyres do not run parallel to the reticle wires, as do the uniaxial ones when rotating the stage: in a perfectly centered optic axis figure the isogyre will rotate like a propeller. There is also some quartz in the rock, smaller, clean light gray grains, but the poikiloblasts are sure of albite. I can also see some biotite and muscovite in the lepidoblastic bands near the top, typical of the facies, also some bluish, high relief epidote grains. I'm curious to know what's the name and probable age of this schist unit, and where is located in Colombia, since it looks quite similar to those which crop out near Caracas... Also a similar figure with uncrossed polars would be quite welcome. Greetings (saludos) Sebastián Grande. 

it may be CORDIERITE, its reach with inclusion and comes from metapelitic, for determine interference figure you must focus at part without inclusions.

Mohammed 

Reply to Mohammed Majeed Sofyissa: Certainly, cordierite is also a low birefringence mineral, very easy to confuse with plagioclase or quartz, but in this case the color of one of the grains reaches 1st order light yellow, therefore it can't be cordierite, which also in this low grade facies is impossible; besides, cordierite is also biaxial (orthorhombic). If it would have been retrograded cordierite it should have been totally chloritized (pinitized). Instead the crystals are fresh, only filled with helicitic inclusions, so they are of ALBITE. As I told Nathalia Paola before, we have zillions of similar low grade schists in the Cordillera de la Costa, in northern Venezuela, so I'm very familar with them. The albite polikiloblasts reach 2-3 mm in diameter, and can be seen under naked eye in hand samples, which gives a typical mottled aspect to this kind of schists. The metamorphic assemblage is totally of low grade greenschist facies. With regards, Sebastian. 

It seems the proportion of the phase you are interested is major in quantity. A simple way is to record X-ray diffraction (XRD) pattern and confirm the phase by identifying the peaks. This is very quick way to identify phases. Please note that, XRD peaks are observed for crystalline phases only and the proportion of the phase must be more than 5% (depending upon their intensity of diffracted beam) and the sample should be in powder form.

The grey mineral if uniaxial +ve it must be quartz.  Feldspars are biaxial -ve mostly, except for sanidine that has a very low optic axial angle and may sometimes appear to be uniaxial but -ve not +ve.  Cordierite is also biaxial and has a much higher refractive index and thus will show much higher relief.

I'm really thankful for all of your responses, I really appreciate them. Indeed it's a plagioclase, it is part of the Conjunto Metamórfico Etpana in la Guajira, it also has garnets so it might be anfibolite facies, 

Best regards

Nathalia

Hi Nathalia: I'm very glad to help a colleague from our brother country Colombia, which geology shares some similarities to Venezuelan geology, at least in the northern parts, being all these units related to the transpressional collision between the Caribbean Plate and Grenada Arc with the NW margin of South America. However, the presence of garnet plus biotite, muscovite, epidote and quartz doesn't indicate garnet amphibolite facies in metapelites, at least there should be staurolite, kyanite or sillimanite for this medium grade facies be reached; and in low P/T gradients even cordierite and/or andalusite (cordierite amphibolite facies). The greenschist facies is subdivided into three subfacies: chlorite, biotite and garnet, but all of them are of low metamorphic grade, where actinolitic amphibole should be present in mafic protoliths. Unless minerals of higher grade are present in the Eptana unit, the facies is still greenschist. If blue-green amphibole or barroisiite is present in metamafites of that unit, together with garnet and epidote, then the facies could be epidote-amphibolite, as defined by Winkler (1978), which is also a low grade facies. "Amphibolite" is a dangerous term to use, since it means a great many different things: it is a rock name used when rocks are composed of plagioclase and amphibole -any plagioclase and any amphibole- in subequal amounts; but it is also the name given to three different metamoprhic facies: garnet, cordierite and epidote-amphibolite. Hence, you should use the term with caution! So the rock you showed us is in the Guajira Peninsula, a real complex area, where rocks range from basement Neoproterozoic Guatupón Gneiss and Jojoncito Leucogranite, to Paleozoic metasediments to Mesozoic ophiolites of Cabo de la Vela to the Paleocene Parashi Tonalite, including a cover of Mesozoic sediments and Jurassic continental rift felsic pyroclastics, and Tertiary sediments too... A similar, but not identical, age sequence can be seen in Paraguaná Peninsula, in NW Venezuela. The Eptana-Jarara Metamorphic Complex should be Mesozoic, related to the Cabo La Vela Ophiolite, quite probably, since it resembles a lot similar Mesozoic-Ophiolitic units in the Cordillera de la Costa, and it should include some metavolcanic lithologies. Do these metamafites contain actinolite or barroisite, by the way? Regards (saludos). Sebastián. 

Dear Nathalia,

Dr. Guenter Grundmann has already hinted that it could be a sodic plagioclase and in a later posting you accepted it also.

However, it is difficult to find the optic sign of a mineral unless the section is at right angles to the optic axis or nearly so. Especially when the mineral is low birefringent exhibiting grey interference colour;,such as in quartz and feldspars; it is quite misleading. In earlier days we were using 'universal stage' for such purposes. 

With best wishes,

R. V. Karanth

I agree that the mineral in the south western which is also on the northern part is plagioclase but cut parallel to {010}. From the habit/form, it could be a Calcic Plagioclase and probably not Sodic.

Regards

Reply to Chinedu Ibe: it can't be calcic plagioclase, the rock is of low metamorphic grade, only albite is possible in the greenschist facies... Calcic plagioclase in metamorphic rocks appears in the highest grades, in the granulite facies. The rock has a typical grenschist facies assemblage: quartz-albite-epidote-muscovite-biotite. Also it is very rare to see twinned low grade sodic plagioclase, very rare indeed, at most it could show a simple Carlsbad twin, but almost never a polysynthetic twinning. But helicitic poikiloblasts are very frequent in albitic plagioclase. Also, having a 1st order grayish inteference color simply says that the mineral is cut very nearly perpendicular to one of the two optic axes. Regards. Sebastian.

Dr. Sebastian Grande,

I do agree with your views.

Regards,

R. V. Karanth