1. Melilite Group X2YZ2O7
By Oliver Boyd

2. Melilite Occurrences
Melilite generally occurs as Calcium Aluminum Inclusions (CAI) in silica and alkali poor igneous rocks.
Barrel-shaped root-beer colored Melilite crystal with clear nepheline. Udersdorf, Rhineland-Palatinate, Germany. Photomicrograph by Lou Perloff / Photo-Atlas of Minerals

3. Melilite Physical Properties
Cleavage: [001] Distinct, [100] Indistinct
Color: white, yellowish white, brown, gray, or greenish gray.
Density: , Average = 2.95
Diaphaniety: Translucent
Fracture: Uneven - Flat surfaces (not cleavage) fractured in an uneven pattern.
Habits: Massive - Lamellar - Distinctly foliated fine-grained forms., Granular - Generally occurs as anhedral to subhedral crystals in matrix.
Hardness: Apatite-Knife Blade
Luminescence: None.
Luster: Vitreous - Greasy
Streak: white
Dichroism (e): colorless or light yellow.
Dichroism (w): colorless or dark yellow.
Optical Data: Uniaxial (-),
e= ,
w= ,
birefringence=
From

4. Melilite Structure End-Member Na-Melilite Gehlenite Akermanite
Formula CaNaAlSi2O7 Ca2Al(Al,Si)2O7 Ca2MgSi2O7
Form.Wt Z
Crystal System Tetr Tetr Tetr
Point Group -42m -42m -42m
Space Group P-421m P-421m P-421m
Unit Cell Tetragonal a = b  c; a = b = g = 90º
a(Å)
c(Å)
Vol(Å3)
Molar Vol
Density
Thermal Expansion(Volumetric)
alpha a a
From

5. Melilite Structure (Gehlenite)
Atoms x y z
X(Ca,Na) ½-x
Y(Al,Mg)
Z(Al,Si) ½-x O
O ½-x O
From Louisnathan (1970)

6. Melilite Chemistry X2YZ2O7
X - 8 Coordination Sites
Sr, Na and K can substitute for Ca.
Y - Tetrahedral Sites
Mn, Co, Zn, Si, B, Be, Al, Fe2+ and Mg are found in these sites.
Z – Tetrahedral Sites
Be, Al and Si are found in these sites.

7. Melilite Elasticity
From Li et al. (1990)