| Johnsenite-(Ce) | |
| Category: | Cyclosilicate |
| Formula: | (original form) |
| Imasymbol: | Jsn[1] |
| Strunz: | 9.CO.10 (10 ed) 8/E.25-57 (8 ed) |
| Dana: | 64.1.2.7 |
| System: | Trigonal |
| Class: | Hexagonal scalenohedral (m) H-M symbol: (2m) |
| Symmetry: | Rm |
| Unit Cell: | a = 14.24, c = 30.03 [Å] (approximated); Z = 3 |
| Color: | Pale yellow to bright orange |
| Habit: | Skeletal crystals (etched); aggregates |
| Cleavage: | None |
| Fracture: | Uneven |
| Tenacity: | Brittle |
| Mohs: | 5–6 |
| Luster: | Vitreous |
| Streak: | White |
| Diaphaneity: | Transparent or translucent |
| Density: | 3.24 (measured) |
| Opticalprop: | Uniaxial (−) |
| Refractive: | nω = 1.65, nε = 1.64 (approximated) |
| Pleochroism: | None |
Johnsenite-(Ce) is a very rare mineral of the eudialyte group,[2] with the chemical formula .[3] The original formula was extended to show the presence of both the cyclic silicate groups and silicon at the M4 site, according to the nomenclature of the eudialyte group.[4] It is the third eudialyte-group mineral with essential tungsten (after khomyakovite and manganokhomyakovite), and second with essential rare earth elements (after zirsilite-(Ce), which is the niobium-analogue of johnsenite-(Ce)). In fact, some niobium substitutes for tungsten in johnsenite-(Ce). Other characteristic feature is the presence of essential carbonate group, shared with carbokentbrooksite, golyshevite, mogovidite and zirsilite-(Ce).
Johnsenite-(Ce) was discovered in alkaline rocks of Mont Saint-Hilaire, Quebec, Canada, which is also a type locality for other eudialyte group species: oneillite, khomyakovite and manganokhomyakovite.[5] The association of johnsenite-(Ce) is rich, as it includes aegirine, albite, amphibole-group mineral, burbankite-group mineral, calcite, catapleiite, cerite-(Ce), dawsonite, epididymite, fluorapophyllite, galena, microcline, molybdenite, natrolite, pectolite, pyrite, pyrrhotite, quartz, rhodochrosite, sphalerite, steacyite, stillwellite-(Ce), titanite, tuperssuatsiaite, zakharovite and zirsilite-(Ce).
Notable impurities in johnsenite-(Ce) are iron, titanium, niobium, yttrium, potassium, praseodymium, and neodymium. Traces of gadolinium, samarium, and hafnium are also reported.
Johnsenite-(Ce) has four-layer structure.