Gmelinite

Gmelinite-Na
Gmelinite from Flinders, Victoria, Australia. Specimen size 2.8 cm.
General
Category	Zeolites
Formula (repeating unit)	Na4(Si8Al4)O24·11H2O
Strunz classification	9.GD.05 (10 ed) 8/J.26-50 (8 ed)
Dana classification	77.1.2.6
Crystal system	Hexagonal
Crystal class	Dihexagonal dipyramidal (6mmm) H-M symbol: (6/m 2/m 2/m)
Space group	P63/mmc
Unit cell	a = 13.78, c = 10.03 [Å]; Z = 4
Identification
Formula mass	2,000.77 g
Color	Colorless, white, yellow, orange, pale green, pink, red, brown and grey
Crystal habit	Hexagonal plates, or short prisms, showing hexagonal dipyramids, pyramids and basal pinacoid. {1010}, {1011} and {0001} dominant. May also be tabular or rhombohedral. Crystals are striated parallel to (0001)
Twinning	Interpenetrant twins common on {1011}. The twins consist of four individuals, three are at 90° to the other and at 60° to each other
Cleavage	Good on {1010}
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	4½
Luster	Dull to vitreous
Streak	White
Diaphaneity	Transparent, translucent or opaque
Specific gravity	2.04 to 2.17
Optical properties	Uniaxial (-)
Refractive index	nω = 1.476 - 1.494, nε = 1.474 - 1.480
Birefringence	δ = 0.002 - 0.014
Solubility	Soluble in cold 10% HCl.
Other characteristics	Piezoelectric. Barely detectable radioactivity. As with all zeolites, water is released on heating, and almost all has been expelled by 400 °C.
References

Gmelinite-Na is one of the rarer zeolites but the most common member of the gmelinite series, gmelinite-Ca, gmelinite-K and gmelinite-Na. It is closely related to the very similar mineral chabazite. Gmelinite was named as a single species in 1825 after Christian Gottlob Gmelin (1792–1860) professor of chemistry and mineralogist from Tübingen, Germany, and in 1997 it was raised to the status of a series.
Gmelinite-Na has been synthesised from Na-bearing aluminosilicate gels. The naturally occurring mineral forms striking crystals, shallow, six sided double pyramids, which can be colorless, white, pale yellow, greenish, orange, pink, and red. They have been compared to an angular flying saucer.

The aluminosilicate framework is composed of tetrahedra linked to form parallel double six-membered rings stacked in two different positions (A and B) in the repeating arrangement AABBAABB. The framework has no Al-Si order. Within the structure there are cavities with a cross-section of up to 4 Å, and also wide channels parallel to the c axis with a diameter of 6.4 Å. Space group: P6₃/mmc. Unit cell parameters: a=13.72 Å, c=9.95 Å, Z=4.

Generally occurs in Si-poor volcanic rocks, marine basalts and breccias, associated with other sodium zeolites such as analcime,
Na(Si₂Al)O₆·H₂O, natrolite, Na₂(Si₃Al₂)O₁₀·2H₂O, and chabazite-Na, Na₂Ca(Si₈Al₄)O₂₄·12H₂O. It also occurs in Na-rich pegmatites in alkaline rocks, and as an alteration product in some nepheline syenite intrusions. No sedimentary gmelinite has been found. It is generally assumed that it forms at low temperatures, less than 100 °C. It is widespread as a hydrothermal alteration product of ussingite, Na₂AlSi₃O₈(OH), associated with gobbinsite, Na₅(Si₁₁Al₅)O₃₂·11H₂O, gonnardite, (Na,Ca)₂(Si,Al)₅O₁₀·3H₂O, and chabazite-K.