Dear Mr. Gonzalez,

the refractive indices are too high to be called a tourmaline. Did you check the pleochroism ? It is so strikingly high for toumaline that you can take a decision by yourself after having tested it. After all, the outward appearrance in terms of optical and morphological features point to sphene. Yet at  this level, as far as the optical properties are concerned, there are some other so-called semi-opaque minerals. For a precise determination it might even be of assistance to describe the lithology in more detail and the environment where you  took the specimen from. I do not like to hand down a verdict, if some sound and easy-to-achieve proofs have not been presented.

Best regards

H.G.Dill

Do you see any pleochroism ? Compare also page 71 and 72 of the appended link.

Regards 

http://dmg-home.de/pdf/Guide_%20reduced%20resolution.pdf

how about extinction? parallel? 

From my understanding tourmaline can appear in a variety of shapes under light microscopy. 

Yes, it can manifest as the striking crystal spray structure:

http://www.brocku.ca/earthsciences/people/gfinn/minerals/tourmal.htm

But also manifest as shapes much more similar to your specimen:

http://rbmason.ca/databank/mineral/tourmaline.html

I would perhaps suggest trusting your other ID factors such as pleochroism, conchoidal fractures parallel extinction rather than having shape as a final accept or decline for mineral ID. Particularly if only using a light microscope. 

Hope this helps. 

Adrienne

Morphology looks compatible with tourmaline, but, as alredy suggested by Dominique Ectors, more information is necessary. Absence of pleochroism would exclude tourmaline. EDS/WDS analysis would be very helpful.

What's the size of that crystal? Looks like a very high magnification image.

high relief and interference colors may also indicate sphene, if extinction angle is not 0, but this is only a photo (sem with eds will give you a prompt answer)

looks like titanite (sphene) to me

The relief and color of the mineral suggest sphene (titanite) more than tourmaline

Dear Mr. Gonzalez,

the refractive indices are too high to be called a tourmaline. Did you check the pleochroism ? It is so strikingly high for toumaline that you can take a decision by yourself after having tested it. After all, the outward appearrance in terms of optical and morphological features point to sphene. Yet at  this level, as far as the optical properties are concerned, there are some other so-called semi-opaque minerals. For a precise determination it might even be of assistance to describe the lithology in more detail and the environment where you  took the specimen from. I do not like to hand down a verdict, if some sound and easy-to-achieve proofs have not been presented.

Best regards

H.G.Dill

It looks to be a monazite, rather than a tourmaline.

To get back on the topic. I agree with many above. It looks like Sphene. Once again, what is the magnification?

This is titanite (sphene).

I agree it's a sphene

I also think that it is a sphene

It is Tourmaline because: (1) the 6-sided shape is characteristic (ref. Rutley's Elements of Mineralogy, p.377: Fig.136: Difference between tourmaline and sphene) (2) Sphene occurs as 4 sided rhombic crystals/lozenge shaped, (3) The crystal is not in extinction position, but there is a slight change in colour between the two figs. In case of sphene, change of colour is unlikely.

It is difficult to determine from photos, however, I'd rather suppose, it is sphene. The birefringence and refractive indices seem to be too high for tourmaline (birefringence of the latter ca. 0.016, sphene ca. 0.100). The roughness of the surface is visible very well, too well for tourmaline. What about pleochroism?  How is the light extinction? Tourmaline shoulf have it paralel to its elongation.

the present photos not tourmaline and not sphene (titanite) but it is allanite 

I mentioned, that photos are not a good basis to mineral determinations. I agree, that allanite may be considered, though it has lower birefringence (0.018-0.031), it is brown or green in thin section and frequently (mostly) metamictised. thus isotropic when seen between crossed polarizers. The photos do not show such features. One remark to the B. Nayak's answer: the upper photo presents the mineral between crossed polarizers, the lower one - with one polarizer, thus the colours of the two images of the discussed grain cannot be compared.

could be allanite 

No, this crystal cannot be tourmaline,  because of its color and interference color.

Another feature: allanite usually contains radioactive elements in sufficiently high concentrations to form pleochroic rims in biotite. Apparently biotite contacts the discussed grain but without any such rim (lower photo). However, if we give no evidences to suport our opinion, this grain "could be" just anything..

No, I am not sure. Too high interference for tourmaline. You can check adsorbtion scheme. Tourmaline has very strong reverse adsorption scheme. You can compare your grain with common biotite. Biotite has direct adsorbtion scheme. 

Hi David

This is titanite (sphene) because of birefringence color and high  relief. What is host rock? Is it a granitic rock? 

it's very possible. More - titanit very typical for wide range of different metamorphic and magmatic rocks. But there are some another accessories, which have a similar features with titanite.

Certainly not tourmaline, because the refractive index is too high. So is also the birefringence; most probable candidates is titanite as many of the comments above state. For monazite the birefringence is too high, as it is also for zircon and xenotime, which additionally have straight extinction - apparently not the case here. It is difficult (if not in all impossible) and therefore not advisable to identify minerals from two pictures.

No, it may not be tourmaline. I have not seen like this one. It may be alanite?? Confirm the grain by EPMA.

Thanks everybody for your answers.... sorry  I missed it...., in Streckeisen diagram the rock plots in the field of  Granite - just in the line that divides  syeno-granite and monzo-granite. Biotite  is the only ferro-magnesian silicate and is more abundant than muscovite . The rock outcrops as a 10m porfiritic dike with plagioclase phenocrysts and a Qtz-Kfs and less plagioclase matrix.....this mineral forms between 1% and 2% of the rock volume.

The magnification is 10X.... it has no pleochroism...the color is just the light brow that is seen there. The extinction angle is 0.

So it isn't tourmaline..... And what about zircon? I've never seen sphene or zircon in a thin section, just in photos that show very nice crystals of them...

Dear Mr. Gonzalez,

since no pleochroism was observed, you can definitely delete tourmaline from your list of minerals in question. As you mentioned a straight extinction, sphene which is monocline, is a matter of debate, particularly in a section like that. You should also take into consideration other minerals such as minerals of tetragonal crystallographic structure such as cassiterite or rutile, even if their optical data are different in my opinion. Zircon would not be my first option because there are biotite flakes nearby devoid of any pleochroic haloes. What age of formation do you presume for the magmatic rock?

Best regard

H.G.Dill

That's a good point Dr, Harald.... a small intrussion associated to this dike was dated with K-Ar about 50 Ma a regional mapping but intrussions have never been studied before, so  I decided to do an exploratory research that consists in mapping the Villonaco Mountain Area in the southern Cordillera Real in the Andes and describe the lithologies found there.... This one of biggest dikes found there. The surronding rocks are shists that came from devonian sediments and then suffered metamorphism but the age of this rock unknown

.

Rocks enjoy metamorphism, they do not suffer it. If I may.

Thanks Giulio!!! I's just that the spanish word for suffer is the one used as a synonim of ''been altered by'' .

it looks like Titanite crystals i've found in Sesia-Lanzo Zone metapelites. the mineralogical assemblage thipically consists of White mica (Phengite), Garnet, Epidote, Zoesite, Clino-Zoesie and Titanite probably deriving from de re-equilibration of Ti-rich micas like Biotite that i found as mineralogical relict of a previous Permian HT assemblage.

I also think that it is a sphene mineral because the body color is masked and this is a characteristic of sphenes.

It looks like sphene

Thanks  Gioele and everybody for you advise, yes the mineral is defenitely titanite. Thank you very much!

It is a Titanite, or sphene crystal that is altered to leucoxene on the margins.

Thank yo Azat!!! I didn´t consider it might be altered.