Mineral Information and Data:

Ca2Al(SiAl)O7 [CNMNC approved formula]

Named after the German chemist, Adolf Ferdinand Gehlen (1775-1815).

IMA status : Valid species (Pre-IMA 1815) Grandfathered
(8th edition) :  
  (55) Sorosilicate Si2O7 Groups, Generally with No Additional Anions
  (55.04) with cations in [8] and lower coordination
  (55.04.01) Melilite group
(10th edition) :
  (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 matrix
Origin: Monzoni, Trento Province, Trentino-alto Adige, Italy
Sample size: 3 x 3.5 cm
Photo: Diederik Visser