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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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Melilite group x 2 yz 2 o 7
Melilite GroupX2YZ2O7

By Oliver Boyd


Melilite occurrences
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
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
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 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

Thermal Expansion(Volumetric)

alpha 24.0

a0 0.2320

a1 0.2679

From http://ruby.colorado.edu/~smyth/min/melilite.html


Melilite structure gehlenite
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 x 2 yz 2 o 7
Melilite ChemistryX2YZ2O7

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
Melilite Elasticity

From Li et al. (1990)


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