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    Optical Mineralogy: Lab 14 - Fall, 2012 Biaxial Interference Figures

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    sagnik roy
    This document discusses how to determine the optic sign and estimate the 2V angle of biaxial minerals using optical mineralogy. It explains that for biaxial interference figures (BXA), if the gray area between the isogyres turns yellow the mineral is positive, and if inside turns yellow it is negative. For centered optic axis figures, outside yellow is positive, inside yellow is negative. Off-centered figures follow the same rules. Estimating 2V can be done by comparing the separation of features to diagrams relating this to the microscope's numerical aperture and the mineral's b index.

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    Optical Mineralogy: Lab 14 - Fall, 2012 Biaxial Interference Figures

    Uploaded by

    sagnik roy
    26 views13 pages
    This document discusses how to determine the optic sign and estimate the 2V angle of biaxial minerals using optical mineralogy. It explains that for biaxial interference figures (BXA), if the gray area between the isogyres turns yellow the mineral is positive, and if inside turns yellow it is negative. For centered optic axis figures, outside yellow is positive, inside yellow is negative. Off-centered figures follow the same rules. Estimating 2V can be done by comparing the separation of features to diagrams relating this to the microscope's numerical aperture and the mineral's b index.

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    This document discusses how to determine the optic sign and estimate the 2V angle of biaxial minerals using optical mineralogy. It explains that for biaxial interference figures (BXA), if the gray area between the isogyres turns yellow the mineral is positive, and if inside turns yellow it is negative. For centered optic axis figures, outside yellow is positive, inside yellow is negative. Off-centered figures follow the same rules. Estimating 2V can be done by comparing the separation of features to diagrams relating this to the microscope's numerical aperture and the mineral's b index.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    26 views13 pages

    Optical Mineralogy: Lab 14 - Fall, 2012 Biaxial Interference Figures

    Uploaded by

    sagnik roy
    This document discusses how to determine the optic sign and estimate the 2V angle of biaxial minerals using optical mineralogy. It explains that for biaxial interference figures (BXA), if the gray area between the isogyres turns yellow the mineral is positive, and if inside turns yellow it is negative. For centered optic axis figures, outside yellow is positive, inside yellow is negative. Off-centered figures follow the same rules. Estimating 2V can be done by comparing the separation of features to diagrams relating this to the microscope's numerical aperture and the mineral's b index.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
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    of 13

    Optical Mineralogy

    Lab 14 – Fall, 2012


    Biaxial Interference Figures

    1
    Biaxial Sign: Bxa Figures
    • To determine the optic sign of a biaxial
    mineral from a BXA figure, position the
    isogyres so that the melatopes are in the NE
    and SW quadrants
    • There should be an area near the melatopes
    that shows a 1o gray interference color
    • Observe this area as you insert the 550nm
    or 1o red compensator

    2
    Biaxial Sign: Bxa Figures
    • If the 1o gray area in
    region between the
    two isogyres turns
    yellow, the mineral is
    biaxial positive
    • If the 1o gray area
    inside of both the
    isogyres turns yellow
    the mineral is biaxial
    negative
    3
    Biaxial Sign: Centered OA
    Figures
    • Optic axis figures probably provide the easiest
    method for determination of optic sign because
    grains with an orientation that would produce an
    OA figure are perhaps the easiest to find
    • Place the isogyre so that the inside of the isogyre
    is in the NE quadrant
    • Find the area that shows 1o gray close to the
    melatope
    • Observe this area as the 550 nm compensator plate
    is inserted
    4
    Biaxial Sign: Centered OA
    Figures
    • If the area outside of
    the isogyre turns
    yellow, the mineral is
    biaxial positive
    • If the area inside the
    isogyre turns yellow,
    the mineral is biaxial
    negative

    5
    Biaxial Sign: Off-Centered
    Bxa or OA Figures
    • Probably even easier to locate are off-
    centered OA or BXA interference figures
    • Position the isogyre so that it fits best in
    either the NE or SW quadrant
    • Observe the gray area near the melatope and
    note the color change on insertion of the
    550 nm compensator

    6
    Biaxial Sign: Off-Centered
    Bxa or OA Figures
    • If the gray area outside
    the isogyre turns
    yellow, the mineral is
    biaxial positive
    • If the gray area outside
    isogyre turns blue and
    the gray area inside
    the isogyre turns
    yellow, the mineral is
    biaxial negative
    7
    Estimation of 2V
    • Precise determination of 2V can only be
    made by determining the 3 principal
    refractive indices of the mineral
    • 2V can be estimated from Bxa and OA
    figures using the diagrams shown here

    8
    2V Estimation: Bxa Figure
    • For a BXA figure the distance between the
    melatopes is proportional to the 2V angle
    • To estimate the 2V from a BXA figure, it is
    necessary to know the numerical aperture (N.A.)
    of the objective lens used to observe the
    interference figure
    • The microscopes in our labs have an N.A. of
    between 0.65 and 0.85

    9
    2V Estimation: Bxa
    Figure
    • The diagram shown here
    gives a visual estimate of the
    2V angle for objective lenses
    with these two values of N.A.
    for a mineral with a b
    refractive index of 1.6

    10
    2V Estimation: Bxa Figure
    • Remember that if the 2V is 0o the mineral is
    uniaxial, and would thus show the uniaxial
    interference figure
    • The separation of the isogyres or melatopes
    increases with 2V and the isogyres
    eventually go outside of the field of view
    for a 2V of 50o with the smaller N.A., and
    about 60o for the larger N.A
    11
    2V Estimation: Bxa Figure
    • Since the maximum 2V
    that can be observed for a
    Bxa figure depends on the
    b refractive index, the
    chart shown here may be
    useful to obtain more
    precise estimates if the b
    refractive index is known
    As b increases, the maximum or can be measured
    observable 2V decreases

    12
    2V Estimation: OA Figure
    • 2V estimates can be made
    on an optic axis figure by
    noting the curvature of the
    isogyres and referring to
    the diagram shown here
    • Note that the curvature is
    most for low values of 2V
    and decreases to where the
    isogyre essentially forms a
    straight line across the
    field of view for a 2V of
    90o
    13

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