Melilite Group
A group of tetragonal sorosilicates with a disilicate anion (Si2O7)6- or an Al or B-bearing derivative thereof and the general formula given above, where M denotes a small- to medium-sized divalent or trivalent cation (mostly Mg and Al, or rarely Fe, B, Zn, Be, Si, etc) and X is Si, Al or rarely Be or B. In general, Al or B replace one Si atom when M is a trivalent ion, but the charge can also be balanced by coupled substitution of Ca2+ with a monovalent ion, especially Na, and M3+ with M2+, such as in Alumoåkermanite , where Al3+ is still dominant on the M-site but the mineral is far from end-member composition.
In petrology "melilite" usually refers to minerals in the åkermanite -gehlenite series, by far the most abundant members of the group.
A large number of synthetic members are known.
History
Activities related to discovery and approval of the group members
Structural context
Tetragonal System
70 measurement(s) or 98.59%10 member(s) or 71.43%
Orthorhombic System
1 measurement(s) or 1.41%1 member(s) or 7.14%
Unclassified
0 measurement(s) or 0.00%3 member(s) or 21.43%
a5.525—10.912 (7.8014)
b7.1248—17.413 (7.9008)
c4.8177—15.506 (5.5284)
alpha90
beta89.47—115.1 (90.8006)
gamma90
volume244.56—1393.87 (346.454)
Based on 70 measurements
Alumoåkermanite
Bennesherite
Ferri-gehlenite
Gehlenite
Gugiaite
Hardystonite
Hydroxylgugiaite
Justite (of Hofmann-Degen)
Okayamalite
Åkermanite
Chemical context
Stoichiometric formulas
Melilite Group
mindat.org
- March 13, 2023Ca2M(XSiO7)M = Mg, Al, rarely Fe, B, Zn, Be, Si, etc. X = Si, Al, rarely Be or B.
mineralogy.rocks
- Aug. 30, 2022A2B(X2Y7)
Elements recorded on EPMA
66
Ca
66
O
64
Si
49
Al
49
Mg
42
Na
30
Fe
21
K
20
Mn
12
Sr
10
Ti
10
Zn
4
Ge
2
Be
1
B
1
H
Physical properties
ColorColor entities, recognized using a custom trained NER model
4
- colourless
- lemon-colored
3
- cream white
- light brownish white
3
- light yellow
- pale yellowish
2
- light brown
- yellow-brown
2
- greenish grey
1
- pale pinkish
Color noteOriginal color note from the source
Colourless; Colourless, yellow; Cream white; Light brown; Light brownish white, pale greyish-white, whitish, very pale pinkish; Light yellow to lemon-colored; Pale yellowish, white, grey; Yellow-brown, colourless, greenish grey
StreakStreak entities, recognized using a custom trained NER model
9
- grey-white
Streak noteOriginal streak note from the source
White; White, grey-white
Lustre
- Earthy1
- Vitreous5
- Dull1
- Resinous1
- Adamantine1
- Greasy1
Fracture
- Irregular/Uneven2
- Conchoidal1
Transparency
- Opaque1
- Transparent3
- Translucent3
Tenacity
- brittle4
Hardness