| Gonnardite | |
|---|---|
 Gonnardite from the Bundoora Quarry,Victoria, Australia. Specimen size 4.9 cm | |
| General | |
| Category | Tectosilicateminerals,zeolitegroup,natrolite subgroup |
| Formula | (Na,Ca)2(Si,Al)5O10·3H2O |
| IMA symbol | Gon[1] |
| Strunz classification | 9.GA.05 (10 ed) 8/J.21-40 (8 ed) |
| Dana classification | 77.1.5.7 |
| Crystal system | Tetragonal |
| Crystal class | Scalenohedral (42m) H-M symbol: (4 2m) |
| Space group | I42d |
| Unit cell | a = 13.38 c = 6.66 [Å]; Z = 2 |
| Identification | |
| Color | Colorless, white, yellow or pink to salmon orange |
| Crystal habit | Radiating fibrous; massive |
| Mohs scale hardness | 4 to 5 |
| Luster | Vitreous to silky or dull |
| Streak | White |
| Diaphaneity | Translucent |
| Specific gravity | 2.21 to 2.36 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.514, nβ = 1.515, nγ = 1.520 |
| Birefringence | δ = 0.006 |
| 2V angle | Measured, 52° |
| References | [2][3][4][5] |
Gonnardite is a comparatively rare, fibrouszeolite,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 was named in 1896 after Ferdinand Pierre Joseph Gonnard[8] (1833–1923), who was Professor of Mining Engineering at the University of Lyon, France.
Orthorhombic-bipyramidal class 2/m 2/m 2/m andtetragonal-scalenoidal class42m (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
Crystals areprismatic, bounded by {110} and {111} as well as {100} and {001},[7] and gonnardite also occurs as radial hemispheres. Commonly found as zoned prisms oraggregates withthomsonite,natrolite andparanatrolite.[7]
Gonnardite is atectosilicate belonging to the natrolite group. The natrolite minerals are composed of chains of AlO4 and SiO4tetrahedra that link to form frameworks. As with all zeolites, there are channels within the framework, and for the natrolite minerals the channels are occupied bypolyhedra containingsodium,calcium orbarium, 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]
Gonnardite has been found in silica-poor volcanics andpegmatites. It occurs withthomsonite andnatrolite invesicles in thevolcanic rock of The Nut, nearStanley, Tasmania, Australia, intergrown withnatrolite at Don Hill, Tasmania and in drill holes withchabazite andcalcite near Guildford, Tasmania.[11] It is also found innepheline-syenite in theGrenville Geological Province, which is part of theCanadian Shield.[12]Thetype locality (the place where the mineral was first described) is La Chaux de Bergonne,Gignat, Saint-Germain-Lembron, Puy-de-Dôme,Auvergne, France, andtype material from this locality is held at theNatural History Museum, London, England, registration number BM.1930,166.
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