1. Melilite Group X2YZ2O7

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 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

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