Melilite Group X2YZ2O7 By Oliver Boyd

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

Melilite Physical Properties Cleavage: [001] Distinct, [100] Indistinct Color: white, yellowish white, brown, gray, or greenish gray. Density: 2.9 - 3, 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: 5-5.5 - 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=1.626-1.658, w=1.632-1.669, birefringence=0.0060-0.0110. From http://www.webmineral.com/data/Melilite.shtml

Melilite Structure 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 Tetragonal a = 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 Thermal Expansion(Volumetric) alpha 24.0 a0 0.2320 a1 0.2679 From http://ruby.colorado.edu/~smyth/min/melilite.html

Melilite Structure (Gehlenite) 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)

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.

Melilite Elasticity From Li et al. (1990)