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| Formula | Na2(Mg,Fe2+)3(Al,Fe3+)2Si8O22(OH)2 |
| | Optic class & sign | Biaxial positive or negative |
| | Optical orientation | Highly variable:
X near a, Y = b, Z near c (“G” orientation)
X near a, Y near c, Z = b (“C” orientation)
X near c, Y near a, Z = b (“O” orientation)
X near c, Y = b, Z near a (“R” orientation)
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| | Optical plane | (010) for G or R orientation, or Ʇ (010) for C and O orientation |
| | Relief | Moderate to high |
| | Refractive indices | nx = 1.594 -1.702
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ny = 1.612 -1.712
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nz = 1.618 -1.722
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| n increases with increasing Fe2+ for Mg and Fe3+ for Al |
| | Birefringence (max.) | 0.006 - 0.025 |
| | | Δn decreases with increasing Fe2+ along the Glc – Rie join; Δn decreases with increasing Fe3+ along the Glc – MgRie join. Masking of interference colour by mineral colour may occur in strongly coloured varieties. |
| | Optic Angle
| 2Vx
= 90 - 0° |
| | 2Vz
= 0 - 90° |
| | Sign of elongation | Length-slow, l (+), and/or length-fast, l (-), depending on composition |
| | Interference figure | 2V highly variable; very strong dispersion for intermediate compositions very much limits the use of optic axis figures in polychromatic light. |
| | Colour / pleochroism | Colourless (close-to-endmember glaucophane) to strongly coloured and pleochroic in vivid blue and violet
Pleochroism: X = yellow, Y = pale lavender to violet, Z = pale to bright blue (Glc); Z = yellow-brown, Y = dark-gray blue, X = dark blue (Rie); absorption in relation to crystallographical directions: a < b ≈ c (valid for all indicatrix orientations)
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| | Zoning | |
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| Form | Habit | Columnar, long-prismatic, acicular, asbestiform (crocidolite); 4-sided diamond-shaped or 6-sided cross-sections Ʇ c |
| | | Surface | Typically subhedral to euhedral |
| | Cleavage | 2 sets {110} perfect, at 122 and 58° (seen in sections Ʇ c). In prismatic sections, the traces of the two principal cleavage sets are parallel. |
| | Twinning | Simple or multiple on {100}, uncommon |
| | Extinction | Inclined; generally small, c Λ with closest indicatrix axis (X, Y, or Z) = 6 - 10° |
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| Reaction textures | Prograde: replacement of low-pressure mafic minerals; retrograde: replacement of omphacite in eclogite, replacement of Na-amphibole by actinolite |
| | Alteration / decomposition | Relatively stable |
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| Occurence | Ign | Alkaline intrusives (Rie); rare in volcanics |
| | | Met | Glc: blueschists, high-pressure marble; Rie: iron stones, granite gneiss, quartzite, granulite |
| | | Sed | |
| | | Hyd | |
| | | Other | |
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| Distinctive properties | Distinctive colour and pleochroism, habit, including characteristic basal sections, either diamond-shaped or six-sided combination of {110} and {010} faces; characteristic amphibole cleavage |
| | Additional comments | Although petrographically useful, the term ‘crossite’ for intermediate compositions in the glaucophane – riebeckite composition plane has been officially abandoned. |
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