Identifying minerals under the microscope has never been my strong point and, as I discovered in our first metamorphic petrology lab of the year, I could definitely do with a bit of a recap. I made one of my smartest moves yet in this area by deciding to check out a book on mineral identification out of the library (Rocks and Minerals in Thin Section, Second Edition by WS MacKenzie, AE Adams and KH Brodie which I highly recommend!) and it has been of great help. But remembering how to identify minerals is not a fun or easy task, it takes a lot of practice to get your eye in. I have now done a year and a bit of practice and although I wouldn't say I particularly excel in microscope practicals (unless I position myself next to a coursemate who does), I've put together a basic guide to the key minerals out there to help out confused first years who, like me, don't usually have a clue what they are looking at.
Key Definitions:
PPL: Plane Polarised Light - All light waves are vibrating in the same direction
XPL: Cross Polarised Light - Light goes through two polarisers, perpendicular to each other.
Birefringence: The optical property of a mineral which brings about interference colours
Extinction: When a mineral goes black in XPL, when no light can get through the mineral.
Birefringence colours:
Image showing birefringence colours for reference (taken from The Open University ).
How I like to remember them:
1st Order - Black and White to Pale Yellow
2nd Order - Colours you would get on a Rubik's Cube
3rd Order - Pastel Colours/Highlighters
Plagioclase Feldspar
Thought I would ease everyone in so first up is arguably the easiest mineral to spot in a thin section: plag.
I like to remember this one by the fact that it is black and white stripey in XPL.
Chemical Formula: NaAlSi3O8 – CaAl2Si2O8
Colour in PPL: Colourless
Birefringence: 1st order
Extinction Angle: Variable angles, changes with composition
Key determining factor: Distinctive lamellar twinning in XPL (looks very stripey)
Fun Fact: In 2012 the Mars Curiosity Rover analysed a rock that turned out to have high feldspar content.
The black and white stripey minerals seen above are examples of plagioclase feldspar in XPL.
Olivine
Next up is a personal fav of mine, mainly because its one I know how to spot, Olivine.
I like to remember this one by its 2nd Order Birefringence (Rubik's cube colours) in XPL and it's curved cracks.
Chemical Formula: (Mg, Fe)2SiO4
Colour in PPL: Colourless/Pale Brown
Birefringence: Upper 2nd Order (Very high, fun colours)
Extinction Angle: 0
Key determining factor: Distinctive curved cracks and very high relief (looks like its set out from the rest of the minerals)
Fun Fact: In Finland, Olivine rich rocks are used for sauna stones because they are resistant to weathering under repetitive heating and cooling.
Examples of olivine shown in both PPL and XPL
Alkali Feldspar
Having done plag I had to put in k-spar because its another classic. Very pretty and pink in hand specimen, it is how I personally recognise a granite in the field and an all round good vibes mineral.
Chemical Formula: (K,Na)AlSi3 O8
Colour in PPL: Colourless
Birefringence: 1st Order
Extinction: 0
Key determining factor: Simple twinning (black/white)
Fun Fact: The name Feldspar was derived from the German "Feldt" and "Spath" meaning fieldflake, because it was a flakey rock that they kept finding when ploughing their fields.
Very poor quality images (apologies) of kspar in thin section (the mostly black one in XPL).
Clinopyroxene
Clinopyroxene can be easily remembered by its inclined extinction because clino sounds like inclined.
Chemical Formula: Ca(Mg,Fe)Si206
Colour in PPL: Colourless/Pale Green/Brown
Birefringence: 2nd Order
Extinction angle: 40-45
Key determining factor: 2 cleavages at 90 to each other with one dominant, inclined extinction.
Fun Fact: My friends and I like to remember the pyroxenes by their dominant cleavage, which we often make jokes about that don't leave our tutors very impressed...
Clinopyroxene in XPL, illustrating in inclined extinction. The yellow mineral the first image is cpx. The second image shows the stage rotated slightly and the mineral has now gone into extinction.
Orthopyroxene
Of course cpx had to be followed by its slightly more boring (personal opinion due to low birefringence) counterpart, opx.
Chemical Formula: (Mg,Fe)SiO3
Colour in PPL: Grey green to pink (pleiochroic)
Birefringence: 1st Order
Extinction: 0
Key determining factor: 2 cleavages at 90, straight extinction
Fun Fact: As previously stated, opx is a boring mineral. I have scoured the internet for a fun fact and to no avail.
Disclaimer - this photograph is taken from the book A Key for Identification of Rock-Forming Minerals in Thin-Section by Andrew Baker (another great book for mineral ID) because I seem to have lost the one I took myself :(
Quartz
A classic, unproblematic mineral which me and my friends love to identify (even when there isn't actually any present in the thin section oops), you can't really beat a bit of quartz.
Chemical Formula: SiO2
Birefringence: 1st Order
Extinction: 0, can be undulose (sweeps across the mineral as you turn the stage)
Key determining factor: Low relief, all around nothing distinctive apart from when extinction is undulose
Fun Fact: In nature, quartz appears in many different colours apart from blue. Any blue quartz crystals have been artificially coloured.
Quartz in PPL (colourless) and XPL (dull 1st order colours)
Garnet
Going a bit rogue here but I was inspired by the metamorphic practical today and garnet is another easy on to spot once you know how.
Chemical Formula: (Fe,Mg)3Al2Si3O12
Colour in PPL: Colourless
Birefringence: Isotropic (so just goes black in XPL)
Key determining factor: Colourless in PPL, Black in XPL, often appears in the form of rounded porphyroblasts (big crystals)
Fun Fact: Garnet is January's birthstone and is apparently a symbol of long lasting friendship.
Garnet in XPL and PPL, garnet is isotropic so no matter how much you rotate the stage in XPL it will stay extinct.
Biotite
First of the two first year micas, biotite is actually not that hard to identify and a cheeky one that I have found tends to pop up when I don't expect and I have to spend ages going through notes to remind myself what it is.
Chemical Formula: K(Mg,Fe)3AlSi3O10(OH,F)2
Colour in PPL: green to brown (pleiochroic)
Birefringence: 3rd Order
Extinction: 0
Key determining factor: Brown, pleiochroic
Fun Fact: The largest crystal of biotite ever found was in Iveland, Norway and was about 7 sq m.
Biotite in XPL and PPL (the brown mineral).
Muscovite
The other mica, which i'm pretty sure I have never knowingly seen in thin section until earlier today but not going to lie, looks pretty cool, is muscovite.
Chemical formula: KAl3Si3O10(OH,F)2
Colour in PPL: Colourless (is also known as white mica)
Birefringence: 3rd Order
Extinction: 0
Key determining factor: Very bright in XPL, often elongated in shape so look for colourful ribbon type shapes.
Fun Fact: In the 14th century, Muscovite was used as a class substitute in Russia, hence the name Muscovite comes from Muscovy State in Russia
Muscovite is the long thin mineral that is brightly coloured in XPL.
Kyanite
Thought I would throw in another metamorphic mineral here with my favourite aluminosilicate polymorph: Kyanite. Looks like garnet until you pop it in XPL.
Chemical Formula: Al2SiO5
Colour in PPL: Colourless
Birefringence: 1st Order
Extinction: 0.5
Key determining factor: Looks like garnet but isn't isotropic.
Fun Fact: Kyanite, along with Andalusite and Silimanite are polymoprhs of Al3SiO5 meaning that they have the same chemical composition but different structures. Different polymoprhs will form under different conditions of temperature and pressure.
Kyanite in PPL (grey mineral) and XPL (colourful one)
Hornblende
Had to put in a cheeky amphibole, one that I personally managed to avoid for the whole of my first year until some in the field was asked to name the minerals in a granite and for some reason started with hornblende. A rouge decision from them when quartz and kspar usually pop to mind, but nevertheless a mineral I have not forgotten since.
Chemical Formula: (Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22 (OH)2
Colour in PPL: Yellow green to dark brown (pleochroic)
Birefringence: 2nd order (often masked)
Extinction: 34
Key determining factor: 34 degree extinction (How I like to tell it apart from biotite because I confuse the two sometimes). Often comes in diamond shapes.
Fun Fact: Hornblende comes from the German word 'blenden' meaning to dazzle or blind, probably refering to its brilliant luster.
Amphibole is the diamond shaped mineral shown above in PPL and XPL.
More fun facts, along with the ones used here, an be found at https://kids.kiddle.co/ https://www.kidzrocks.com/ and https://topfacts.org/
Mineral photos used were taken by me in the lab (top tip for taking photos down your microscope, which is annoyingly hard to do, use your other hand flat against the eyepiece to steady the camera.)
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