Gehlenite
Ca2Al(SiAl)O7 [CNMNC approved formula]
Named after the German chemist, Adolf Ferdinand Gehlen (1775-1815).
| IMA status : | Valid species (Pre-IMA 1815) Grandfathered |
| CLASSIFICATION | |
| Dana (8th edition) : |
55.04.01.02 |
| (55) Sorosilicate Si2O7 Groups, Generally with No Additional Anions | |
| (55.04) with cations in [8] and lower coordination | |
| (55.04.01) Melilite group | |
| Nickel-Strunz (10th edition) : |
09.BB.10 |
| (09) Silicates | |
| (09.B) Sorosilicates (dimer) structures | |
| (09.BB) Si2O7 groups, without non-tetrahedral anions; cations in tetrahedral [4] and greater coordination | |
| Crystal system: | Tetragonal System |
| Point group (H-M): | 42m (or 4 2m) — scalenohedral |
| Space group: | P4 21m |
| Unit cell: | a = 7.6850(4) Å, c = 5.0636(3) Å, Z=2 |
| Crystal Habit: | Crystals are commonly short prismatic, resembling octahedrally modified cubes; granular, massive. |
| Twinning: | On {100}, {001}; lamellar on {001} |
| Color: | Colorless, brown, yellowish, greyish green; colorless to pale yellow in thin section |
| Diaphaneity: | Transparent to translucent and opaque |
| Luster: | Vitreous to resinous |
| Hardness (Mohs): | 5 - 6 |
| Measured Density: | 3.038 g/cm3 |
| Cleavage: | Distinct on {001}, poor on {110} |
| Tenacity: | Brittle |
| Fracture: | Uneven, splintery to conchoidal |
| Streak: | White to grayish white |
| Polymorphism and Series: | Forms a series with akermanite. |
| Geologic Setting: | In contact metamorphosed impure limestones; in calcium-rich ultramafic volcanic rocks. |
| Mineral Association: | Dolomite, calcite, augite, olivine, nepheline, leucite, merwinite, spurrite, diopside, ferroan spinel, phlogopite, pyrope, grossular, antigorite, pyroxenes, apatite, vesuvianite |
Dark grey-brown tetragonal crystals up to 5 mm in matrixOrigin: Monzoni, Trento Province, Trentino-alto Adige, Italy
Sample size: 3 x 3.5 cm
Photo: Diederik Visser