| Id | 22 |
| Identification Type | Mineral |
| Name | Biotite |
| Distinguishing Features | Flaky
Pseudo-isotropic Pleochroic bird's-eye-extinction |
| Description | Occurs as flakes larger than surrounding material, green to brown transparent, may be pleochroic. Low to moderate relief. Cleavage planes frequently visible as platy layers. Classic bird's eye extinction (mottled, pebbly texture) not always present in smear slide. Very low apparent birefringence (pseudo-isotropy) because flakes settle with {001} in the plane of the slide. However, by tilting the slide very slightly (e.g., by slipping another slide under the slide) one can evaluate the true birefringence (thanks to Frank Brown, U. Utah, for the tip). Biotite has strong birefringence; chlorite, which may form as an alteration product or a metamorphic mineral, can look similar to biotite in plane light but has weak birefringence. |
| Tags | brown green layers ragged silicate mica transparent |
| External Resources | Biotite on MinDat |
| Imposters |
Chlorite
Biotite can be altered to Chlorite. Birefringence can be used to distinguish, but is not obvious when mica flakes settle on the slide with {001} parallel to the slide. To see birefringence colors, slightly incline the slide by slipping another slide under it. Biotite is strongly birefringent; chlorite weakly |
| Images |
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Unique Identification: Biotite Site name: Big Soda Light type: Plane-Polarized Light Magnification: 400x Submitted by: Amy Myrbo Notes: calcite-inclusion, Image Tags: none |
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Unique Identification: Biotite Site name: Big Soda Light type: Cross-Polarized Light Magnification: 400x Submitted by: Amy Myrbo Notes: calcite-inclusion, Image Tags: none |
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Unique Identification: Biotite Site name: not specified Light type: Plane-Polarized Light Magnification: 400x Submitted by: Amy Myrbo Notes: none Image Tags: none |
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Unique Identification: Biotite Site name: not specified Light type: Plane-Polarized Light Magnification: 630 Submitted by: Michael Rosen Blas Valero Notes: colored green platey heavy-mineral basal-section Image Tags: none |
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Unique Identification: Biotite Site name: not specified Light type: Cross-Polarized Light Magnification: 630 Submitted by: Michael Rosen Blas Valero Notes: colored green platey heavy-mineral basal-section Image Tags: none |
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Unique Identification: Biotite Site name: not specified Light type: Plane-Polarized Light Magnification: not specified Submitted by: Guy Rothwell Notes: Large biotite mica flake, note the conspicuously large size of the flake compared with the associated material, flakes Image Tags: none |
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Unique Identification: Biotite Site name: not specified Light type: Plane-Polarized Light Magnification: not specified Submitted by: Guy Rothwell Notes: Field of brown and green mica flakes, pyroxene crystal fragments also in the field, pyroxenes crystal-fragment flakes Image Tags: none |
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Unique Identification: Biotite Site name: Bengal Fan, Indian Ocean ( 8 00.42 N, 86 16.97 E) Light type: Plane-Polarized Light Magnification: not specified Submitted by: Guy Rothwell Notes: Field of green and brown mica flakes, flakes Image Tags: none |
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Unique Identification: Biotite Site name: not specified Light type: Plane-Polarized Light Magnification: not specified Submitted by: Guy Rothwell Notes: Brown biotite mica flake, flakes Image Tags: none |
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Unique Identification: Biotite Site name: not specified Light type: Plane-Polarized Light Magnification: not specified Submitted by: Guy Rothwell Notes: Large green biotite mica flake, flakes Image Tags: none |
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Unique Identification: Biotite Site name: Yellowstone Light type: Plane-Polarized Light Magnification: 400 Submitted by: Val Stanley Notes: A thin book of biotite sheets, with torn edges Image Tags: flaky brown |
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Unique Identification: Biotite Site name: Reid Lake Light type: Plane-Polarized Light Magnification: 400 Submitted by: Val Stanley Notes: Pleochroic halo from zircon in biotite; discoloration resulting from alpha particle irradiation Image Tags: none |
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Unique Identification: Biotite Site name: Ozera Karakul Light type: Plane-Polarized Light Magnification: 400 Submitted by: Val Stanley Notes: vivid interference colors Image Tags: none |
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Unique Identification: Biotite Site name: Lake Malawi Light type: Plane-Polarized Light Magnification: not specified Submitted by: Andy Breckenridge Notes: biotite, volcanic Image Tags: none |
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Unique Identification: Biotite Site name: Lake Malawi Light type: Cross-Polarized Light Magnification: not specified Submitted by: Andy Breckenridge Notes: biotite, volcanic Image Tags: none |
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Unique Identification: Biotite Site name: Mubwindi wetland Light type: Scanning Electron Micrograph Magnification: 7000.0 Submitted by: Mark Shapley Notes: Biotite; composition by EDX. Note platey phyllosilicate structure; in a smear slide, biotite grains will typically be viewed perpendicular to the cleavage plane, making this structure difficult to see. Image Tags: mica platey sheet good-cleavage SEM |
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