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%
a5.52510.912 (7.8014)
b7.124817.413 (7.9008)
c4.817715.506 (5.5284)
alpha90
beta89.47115.1 (90.8006)
gamma90
volume244.561393.87 (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