Gmelinite

Gmelinite-Na

Gmelinite from Flinders, Victoria, Australia. Specimen size 2.8 cm.
General
Category Zeolites
Formula
(repeating unit)		 Na4(Si8Al4)O24·11H2O[1]
Strunz classification 09.GD.05
8/J.26-50
Dana classification 77.1.2.6
Crystal system Hexagonal 6/m 2/m 2/m Dihexagonal Dipyramidal
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[2] on {1011}. The twins consist of four individuals, three are at 90° to the other and at 60° to each other[3]
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[2]
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 [4][5]

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.[6]
Gmelinite-Na has been synthesised from Na-bearing aluminosilicate gels.[6] 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.

Structure

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.[3] 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.[7] Space group: P63/mmc. Unit cell parameters:[2] a=13.72 Å, c=9.95 Å, Z=4.

Environment

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.[6] No sedimentary gmelinite has been found.[3] It is generally assumed that it forms at low temperatures, less than 100 °C.[6] 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.[6]

Notable localities

Gmelinite from Ireland

Gmelinite-Na occurs extremely rarely at the Francon Quarry, Montreal, Canada, in sills of the igneous volcanic rock phonolite which are rich in dawsonite, NaAl(CO3)(OH)2.[8] It occurs both as pure gmelinite-Na and interlayered with chabazite in water-quenched basalts in Western Tasmania.[9]

Associated minerals include other zeolites, especially chabazite, quartz, aragonite and calcite.

Distribution

Type Locality: Monte Nero, San Pietro, Montecchio Maggiore, Vicenza Province, Veneto, Italy. Also found in Australia, Canada, Czech Republic, Germany, Hungary, Japan, Russia, UK and US.[5]

References

  1. ↑ http://rruff.info/ima
  2. 1 2 3 Roberts, Campbell and Rapp (1990) Encyclopedia of Minerals, 2nd edition
  3. 1 2 3 Gaines et al (1997) Dana's New Mineralogy Eighth Edition
  4. ↑ http://www.webmineral.com/data/Gmelinite-Na.shtml
  5. 1 2 http://www.mindat.org/min-6973.html Mindat.org
  6. 1 2 3 4 5 Deer, Howie and Zussman (2004) Rock-Forming Minerals Volume 4B:690-696
  7. ↑ Senderov and Shishakova (1967) Russian Chemical Bulletin 16-1:151
  8. ↑ Tarassoff, Peter, and Horvath, Lazlo and Elsa (2006) Mineralogical Record 37-1:35
  9. ↑ Sutherland, F L and Bottrill, R S (2004) Zeolites of Western Tasmania, Australian Journal of Mineralogy 10-2: 59 - 72

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