|
| Formula | (Na,Ca,K)6-8(AlSiO4)6(CO3,SO4,Cl,OH)1-2 ∙ 1-5H2O |
| | Optic class & sign | Uniaxial negative |
| | Relief | Moderate to low negative |
| | Refractive indices | no = 1.490 -1.530
|
|
ne = 1.488 -1.503
|
|
| n decreases with increasing Na and SO42- substitution for Ca and CO32- |
| | Birefringence (max.) | 0.002 - 0.028 |
| | | Δn decreases with increasing Na and SO42- substitution for Ca and CO32- |
| | Sign of elongation | Length-fast, l (-) |
| | Interference figure | |
| | Colour / pleochroism | Colourless |
| | Zoning | |
|
|
| Form | Habit | Short-prismatic to granular |
| | | Surface | Commonly subhedral to anhedral |
| | Cleavage | 3 sets, {1010} perfect; {0001} poor |
| | Twinning | Rare lamellar twins |
| | Extinction | Straight with respect to prism faces and {1010} cleavage in sections ∥ c |
|
|
| Reaction textures | Breakdown product of nepheline; reaction zones between nepheline and calcite |
| | Alteration / decomposition | Replacement by calcite |
|
|
| Occurence | Ign | Nepheline syenites and related alkaline rocks |
| | | Met | |
| | | Sed | |
| | | Hyd | |
| | | Other | |
|
|
| Distinctive properties | Negative relief; Ca-CO3-rich cancrinite shows distinctly higher Δn than nepheline or feldspar |
| | Additional comments | Cl-rich cancrinite (microsommite) is optically positive |
|
|
Images 
