Hornblende vs Biotite under PPL and XPL

When observing igneous rocks,
Intro
Biotite and hornblende are two of the most common dark coloured minerals typically found in igneous rocks. To distinguish between them, we observe them as thin sections under the petrographic microscope.
Hornblende
Under plane-polarised light, hornblende exhibits moderate relief, and cleavages at 60-120 degrees in basal sections. While it does have moderate to strong pleochroism, it is typically masked by it’s strong green to green-brown colour.
As we turn the stage, we can see oblique extinction occurring between 10-25 degrees, and note that basal sections will show symmetrical extinction.
The medium to high birefringence (with interference colours of the upper 1st and lower 2nd order) increase with increasing Fe-content, and depending on oxidisation. The strongly anomalous interference colours are often masked by the strong colour of the crystal.
As for twinning, the green hornblende has simple twinning, while the less common brown hornblende has lamellar twinning.
In hornblende, pleochroic halos may form around radioactive inclusions such as zircon intrusions. Biotite and hornblende may occur together in intermediate plutonic rocks, granites and high-grade rocks. Paragenesis can be used to explain why biotite crystalizes out earlier and this explains why it is sometimes rimmed by hornblende.
Biotite
Under the PPL, biotite displays typical mica habit, cleavage and birefringence but extreme pleochroism, which increases in intensity with increasing iron content, or may be weaker if Mg-rich. It has moderately high relief and platy crystals resemble prisms that show one set of well-developed cleavage traces. However, if the section is orthogonal to this, biotite will appear structureless and brown.
Like hornblende, the interference colours that would be seen under XPL are generally masked, though with only brown - not green. The richer in iron, the stronger the pleochroism tends to be. It has 3rd order birefringence which tends to be dulled by the natural brown colour. Extinciton occurs parallel to the single cleavage.
Unlike hornblende, biotite shows no twinning.
Conlusions
While it may be tedious to identify these minerals in hand specimens, the plane and cross polarisers of the petrographic microscope highlight the fundamental differences between them, allowing clear and confident sample assessments to be made in Earth Science laboratories.

[@kaminijain9249]

Please please please make more thin section videos this helped me out a lot

[@sud-est1295]

thank you

[@abdellahtolba7657]

can you show slides of the symmetrical and oblique Extinction angles

[@jimnorthland2903]

Still don't understand enough to tell what I'm finding. I'll try again when I know more of the lexicon of mineralogy.

[@nicolebrown6382]

Good luck! There's a lot to take in, but if you keep at it, you will be able to put together the full picture.

[@jimnorthland2903]

@@nicolebrown6382 Think I may have a clue. Found where biotite was described as being flaky like mica. What I have is really solid and not at all foliated. So maybe hornblende.

[@nicolebrown6382]

@@jimnorthland2903 Yes, biotite is a platy mineral whereas hornblende is a needle-like mineral, so a lack of foliation indicates hornblende :)

[@jimnorthland2903]

@@nicolebrown6382 Thank you so much for taking the time to reply! I'm sure my samples must be a common stone as I find them regularly. But I am doing something wrong when I am looking for photos online.

[@nicolebrown6382]

@@jimnorthland2903 That's okay! It can definitely be hard to find specific information for mineralogy online

[@cipndale]

Did you get to bathroom in time?

[@datemasamune8863]

may i know what actually is a relief?? kinda confuse right now

[@TARM95]

To my knowledge the relief is how easy the outline of the mineral can be seen in PPL. Minerals like quartz are very low and might look like one big crystal but when in XPL you can see it's lots of different crystals.

[@datemasamune8863]

UnR3ALFX thnx for the explaination

[@TARM95]

Be sure not to confuse the difference in colour for the relief. Quartz is colourless and biotite is brown so it's obvious to see the difference but the relief will actually be the outline of the biotite (usually high) so it will be outlined in black (high relief)

[@datemasamune8863]

UnR3ALFX i see.. need to play more with tge microscope and thin sheets i guess

[@danielknight790]

another way to look at it is this: high relief makes it look like the mineral is jumping off of the thin section at you. Low relief means it has no 3D looking edges or anything like that.

[@BeingAndHappenings]

🧡💛🧡💛