Gonnardite

Gonnardite is a comparatively rare, fibrous zeolite, natrolite subgroup. Older papers claim that a complete solid solution exists between tetranatrolite and gonnardite, but tetranatrolite was discredited as a separate species in 1999.[6] A series, based on the disorder of the silicon-aluminum in the framework, appears to exist between Na-rich gonnardite and natrolite, Na2(Si3Al2)O10·2H2O.[7]

Gonnardite
Gonnardite from the Bundoora Quarry, Victoria, Australia. Specimen size 4.9 cm
General
CategoryTectosilicate
Formula
(repeating unit)		(Na,Ca)2(Si,Al)5O10·3H2O
IMA symbolGon[1]
Strunz classification9.GA.05 (10 ed)
8/J.21-40 (8 ed)
Dana classification77.1.5.7
Crystal systemTetragonal
Crystal classScalenohedral (42m)
H-M symbol: (4 2m)
Space groupI42d
Unit cella = 13.38
c = 6.66 [Å]; Z = 2
Identification
ColorColorless, white, yellow or pink to salmon orange
Crystal habitRadiating fibrous; massive
Mohs scale hardness4 to 5
LusterVitreous to silky or dull
StreakWhite
DiaphaneityTranslucent
Specific gravity2.21 to 2.36
Optical propertiesBiaxial (+)
Refractive indexnα = 1.514, nβ = 1.515, nγ = 1.520
Birefringenceδ = 0.006
2V angleMeasured, 52°
References[2][3][4][5]

Gonnardite was named in 1896 after Ferdinand Pierre Joseph Gonnard[8] (1833–1923), who was Professor of Mining Engineering at the University of Lyon, France.

Crystallography

Orthorhombic-bipyramidal class 2/m 2/m 2/m and tetragonal-scalenoidal class 42m (orthorhombic with a very close to b, or tetragonal with a equal to b).
Unit Cell Parameters: a = b = 13.21 Å, c = 6.622 Å, Z = 2[3][4][9]
Space Group: I42d

Crystal habit

Crystals are prismatic, bounded by {110} and {111} as well as {100} and {001},[7] and gonnardite also occurs as radial hemispheres. Commonly found as zoned prisms or aggregates with thomsonite, natrolite and paranatrolite.[7]

Structure

Gonnardite is a tectosilicate belonging to the natrolite group. The natrolite minerals are composed of chains of AlO4 and SiO4 tetrahedra that link to form frameworks. As with all zeolites, there are channels within the framework, and for the natrolite minerals the channels are occupied by polyhedra containing sodium, calcium or barium, together with oxygen and water.[10] Gonnardite has the same framework structure as natrolite, but a disordered Si, Al distribution on the tetrahedral sites.[9] Some of the water sites in the disordered natrolite structure of gonnardite are empty.[7]

Environment

Gonnardite has been found in silica-poor volcanics and pegmatites. It occurs with thomsonite and natrolite in vesicles in the volcanic rock of The Nut, near Stanley, Tasmania, Australia, intergrown with natrolite at Don Hill, Tasmania and in drill holes with chabazite and calcite near Guildford, Tasmania.[11] It is also found in nepheline-syenite in the Grenville Geological Province, which is part of the Canadian Shield.[12] The type locality (the place where the mineral was first described) is La Chaux de Bergonne, Gignat, Saint-Germain-Lembron, Puy-de-Dôme, Auvergne, France, and type material from this locality is held at the Natural History Museum, London, England, registration number BM.1930,166.

References
  1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. "Gonnardite".
  3. "Gonnardite Mineral Data".
  4. Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C. (2005). "Gonnardite" (PDF). Handbook of Mineralogy. Mineral Data Publishing. Retrieved 13 March 2022.
  5. Gaines et al (1997) Dana's New Mineralogy, Wiley
  6. American Mineralogist (1999) 84: 1445-1450
  7. Tschernich, Zeolites of the World (1992) Geoscience Press, pages 215 to 225
  8. "Archived copy". Archived from the original on 2014-07-29. Retrieved 2010-03-30.{{cite web}}: CS1 maint: archived copy as title (link)
  9. Mineralogical Magazine (1998) 62: 548
  10. American Mineralogist (1972) 77:685
  11. Australian Journal of Mineralogy (2004) 10-2: 59-72
  12. The Mineralogical Record 37-4: 285
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