Melilite Group X 2 YZ 2 O 7. By Oliver Boyd. Melilite Occurrences. Melilite generally occurs as Calcium Aluminum Inclusions (CAI) in silica and alkali poor igneous rocks.
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By Oliver Boyd
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
Cleavage:  Distinct,  Indistinct
Color: white, yellowish white, brown, gray, or greenish gray.
Density: 2.9 - 3, Average = 2.95
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: 5-5.5 - Apatite-Knife Blade
Luster: Vitreous - Greasy
Dichroism (e): colorless or light yellow.
Dichroism (w): colorless or dark yellow.
Optical Data: Uniaxial (-),
End-Member Na-Melilite Gehlenite Akermanite
Formula CaNaAlSi2O7 Ca2Al(Al,Si)2O7 Ca2MgSi2O7
Form.Wt. 258.219 274.205 272.640
Z 2 2 2
Crystal System Tetr Tetr Tetr
Point Group -42m -42m -42m
Space Group P-421m P-421m P-421m
Unit Cell Tetragonala = b c; a = b = g = 90º
a(Å) 7.6344 7.7173 7.835
c(Å) 5.0513 5.0860 5.010
Vol(Å3) 294.41 302.91 307.55
Molar Vol 88.662 91.220 92.619
Density 2.912 3.006 2.944
Atoms x y z
X(Ca,Na) 0.3375 ½-x 0.5110
Y(Al,Mg) 0.0 0.0 0.0
Z(Al,Si) 0.1431 ½-x 0.9528
O1 0.5 0.0 0.1884
O2 0.1418 ½-x 0.2832
O3 0.0872 0.1706 0.0833
From Louisnathan (1970)
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.
From Li et al. (1990)