|
| Formula | 2Mg2(SiO4) • Mg(OH,F)2 |
| | Optic class & sign | Biaxial positive |
| | Optical orientation | X near c, Y near a, Z = b |
| | Optical plane | Orthogonal to (010) |
| | Relief | Moderate-positive |
| | Refractive indices | nx = 1.592 -1.643
|
|
ny = 1.602 -1.655
|
|
nz = 1.621 -1.676
|
|
| n increases with increasing Fe and Ti, decreases with increasing F |
| | Birefringence (max.) | 0.028 - 0.034 |
| | | Δn increases with increasing Fe and Ti |
| | Optic Angle
| 2Vx
|
| | 2Vz
= 50 - 85° |
| | Sign of elongation | Length-slow, l(+), or length-fast, I(-) |
| | Interference figure | |
| | Colour / pleochroism | Colourless to pale yellow or pale brown, pale green;
pleochroism: X>Y>Z
|
| | Zoning | |
|
|
| Form | Habit | Granular, thick tabular |
| | | Surface | Commonly anhedral |
| | Cleavage | Poor |
| | Twinning | {100} simple or lamellar |
| | Extinction | Generally inclined to traces of crystal faces, if these are developed |
|
|
| Reaction textures | |
| | Alteration / decomposition | Serpentine, chlorite, brucite |
|
|
| Occurence | Ign | Carbonatite, kimberlite |
| | | Met | Impure dolomitic marble |
| | | Sed | |
| | | Hyd | |
| | | Other | |
|
|
| Distinctive properties | Lack of cleavage, colour and pleochroism, restricted occurrence. Optical distinction between the different members of the humite group is difficult. Microprobe or XRD analyses may be necessary. Norbergite, chondrodite, humite: lower n and Δn compared to magnesian olivine. |
| | 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.
Titanochondrodite: 2Mg2SiO4 • Mg1-xTix(OH,F)2-2xO2x
Different members of the humite group may occur as intergrowths, also with olivine or monticellite.
|
|
|
Images 
