|
| Formula | BaCO3 |
| | Optic class & sign | Biaxial negative |
| | Optical orientation | X = c, Y = b, Z = a |
| | Optical plane | (010) |
| | Relief | Low-negative to high |
| | Refractive indices | nx = 1.529
|
|
ny = 1.676
|
|
nz = 1.677
|
|
| n decreases with increasing Sr for Ba |
| | Birefringence (max.) | 0.148 |
| | | |
| | Optic Angle
| 2Vx
= 16° - |
| | 2Vz
|
| | Sign of elongation | Length-fast, l (-), parallel to cleavage traces, but hardly applicable due to very high Δn |
| | Interference figure | Sections ⊥ c yield acute bisectrix figures with numerous isochromes |
| | Colour / pleochroism | Colourless |
| | Zoning | - |
|
|
| Form | Habit | Stubby pseudohexagonal dipyramidal shapes, granular, columnar, fibrous |
| | | Surface | Euhedral to anhedral |
| | Cleavage | {010} |
| | Twinning | {110} always present; cyclic twins with six segments |
| | Extinction | Straight to {010} cleavage traces and prism faces; symmetrical to pyramid faces |
|
|
| Reaction textures | |
| | Alteration / decomposition | Baryte; witherite can be itself an alteration product of baryte |
|
|
| Occurence | Ign | |
| | | Met | |
| | | Sed | Carbonaceous Ba-rich shale and chert |
| | | Hyd | Low-temperature hydrothermal veins and cavities in calcareous sedimentary rocks |
| | | Other | |
|
|
| Distinctive properties | Extreme Δn with high-order white interference colours and marked change of relief with rotation in sections sub-parallel to c, single cleavage, cyclic twinning. Optical properties similar to aragonite. Distinction from other carbonates also possible using staining techniques. |
| | Additional comments | n-Δn chart: Arg - aragonite, Cer - cerussite, Str - strontianite, Wth - witherite |
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