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| Formula | (Mg,Fe)5Si6O16(OH)2 |
| | Optic class & sign | Biaxial negative |
| | Optical orientation | X = a, Y = b, Z = c |
| | Optical plane | (010) |
| | Relief | Moderate-positive |
| | Refractive indices | nx = 1.605
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ny = 1.626
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nz = 1.633
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| | Birefringence (max.) | 0.028 |
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| | Optic Angle
| 2Vx
= 62° - |
| | 2Vz
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| | Sign of elongation | Length-slow, l (+) |
| | Interference figure | Due to fine-scale intergrowths with other multichain silicates, interference figures cannot be easily obtained. |
| | Colour / pleochroism | Colourless |
| | Zoning | |
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| Form | Habit | Long-prismatic; diamond-shaped {001} sections (commonly in combination with other intergrown multichain silicates; see below) |
| | | Surface | Typically subhedral, with well-developed {110} prism faces |
| | Cleavage | 2 sets {110} intersecting at about 142 and 38° (seen in sections ⊥ c). In prismatic sections, the traces of the two principal cleavage sets are parallel. |
| | Twinning | None |
| | Extinction | Straight to prism faces and main cleavage in sections parallel c. Symmetrical to {110} cleavage and prism faces in {001} sections. |
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| Reaction textures | |
| | Alteration / decomposition | |
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| Occurence | Ign | |
| | | Met | Amphibolite-facies ultramafic rocks, metasomatic in blackwall rocks, cordierite-orthoamphibole rocks |
| | | Sed | |
| | | Hyd | |
| | | Other | |
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| Distinctive properties | Multichain pyriboles involving triple chains are typically occurring as (010)-parallel lamellar intergrowths of chesterite, jimthompsonite and anthophyllite. (100)-parallel lamellae of monoclinic varieties such as cummingtonite and clinojimthompsonite may also be present.
The intergrowths are indicated by a narrow-spaced stripy appearance on (001) sections showing variable interference colours for the different domains.
Optical distinction of different pyriboles in intergrowths is difficult, though. Crystal domains large enough to allow observation of the {110} cleavage or crystal face intersections may reveal the distinct 54° anthophyllite, 45° chesterite and 38° jimthompsonite angles.
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| | Additional comments | The expected variation of optical properties in these pyribole solid solutions is not well documented.
The single-crystal diagram is schematic as pyribole types are typically intergrown at a fine scale with other multichain silicates.
Clinojimthompsonite is the monoclinic variety (c∧Z = 10°) with similar refractive indices and birefringence.
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