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| Formula | 4Mg2SiO4 • Mg(OH,F)2 |
| | Optic class & sign | Biaxial positive |
| | Optical orientation | X near c, Y near a, Z = b |
| | Optical plane | Orthogonal to (010), near (001) |
| | Relief | Moderate to high |
| | Refractive indices | nx = 1.628 -1.702
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ny = 1.641 -1.708
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nz = 1.662 -1.728
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| n increases with increasing Fe and Ti, decreases with increasing F |
| | Birefringence (max.) | 0.028 - 0.041 |
| | | Δn increases with increasing Fe and Ti |
| | Optic Angle
| 2Vx
|
| | 2Vz
= 52 - 85° |
| | Sign of elongation | Length-slow, l(+) or length-fast I(-) |
| | Interference figure | |
| | Colour / pleochroism | Colourless to pale yellow, golden yellow, yellow-brown, or orange; colour intensity increases with Fe and Ti; pleochroism: X>Y=Z
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| | Zoning | |
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| Form | Habit | Granular, tabular |
| | | Surface | Commonly anhedral |
| | Cleavage | Poor |
| | Twinning | {100} simple or lamellar |
| | Extinction | Generally inclined to traces of crystal faces, if if they are developed |
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| Reaction textures | |
| | Alteration / decomposition | Serpentine, chlorite, brucite |
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| Occurence | Ign | Carbonatite, kimberlite |
| | | Met | Impure dolomitic marble, serpentinite, talc schist |
| | | Sed | |
| | | Hyd | |
| | | Other | |
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| Distinctive properties | Colour and pleochroism, lack of cleavage, restricted occurrence. Optical distinction between the different members of the humite group is difficult. Microprobe or XRD analyses may be necessary. |
| | Additional comments | Humite group minerals tend to form where plutonic rocks have introduced F into carbonate wall rocks. Norbergite, chondrodite and humite in particular contain substantial amounts of F, with contents increasing from clinohumite to norbergite.
Titanoclinohumite: 4Mg2SiO4 • Mg1-xTix(OH,F)2-2xO2x
Different members of the humite group may occur as intergrowths, also with olivine or monticellite. |
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