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Amygdaloidal is a name given to igneous rocks, usually old lava flows, full of almond-shaped cavities which have been filled up with secondary minerals, such as calcite, agate or the zeolites. These cavities vary in size up to several centimetres across and were formed while the rock was still fluid and in motion.
 Research Amygdaloidal
Analcime is a mineral of the zeolite group noted for its vitreous lustre, and has the formulae NaAlSi2O6ù2H2O and a relative hardness of 6. It is found in the cavities of intrusive and volcanic igneous rocks; often as clear shiny crystals and is associated with calcite and other zeolites. Analcime was first identified on islands off Sicily and was confirmed as a distinct mineral in 1797 by the French mineralogist Rene-Just Hauy.
Research Analcime
Gyrolite is a white or colourless, glassy-looking secondary mineral. A silicate of calcium, gyrolite is formed through the alteration of pre-existent calcium silicates and is found mainly in crevices of basalt or as white spherules on a basalt groundmass, rarely larger than 5 cm in diameter. Gyrolite was first discovered in 1851 and was confirmed as a distinct mineral in 1855. Gyrolite aggregates are often accompanied by apophyllite, quartz and zeolites such as analcime.
Research Gyrolite
Pectolite is a brittle mineral, a hydrous silicate of sodium and calcium with the formulae NaCa2Si3O8(OH) and a relative hardness of 5. It is formed from hydrothermal solutions filling cavities in basalts. Pectolite was confirmed as a distinct species in 1828 and is associated with zeolites, prehnite and calcite.
Research Pectolite
Prehnite is a mineral named after Colonel Hendrick Von Prehn who first discovered it at the Cape of Good Hope in South Africa. It was confirmed as a distinct species of mineral in 1788. Prehnite is a hydrous silicate of calcium and aluminium with the formulae Ca2Al2Si3O10(OH)2 and a relative hardness of 6 to 7. Prehnite occurs as a crusty lining in cavities in basalt and related rocks and is associated with zeolites, datolite, pectolite, and calcite. Prehnite has a characteristic green colour. Resembles hemimorphite but is of a lower specific gravity and fuses easily.
Research Prehnite
Zeolites are a group of complex silicate minerals mostly occurring in cavities in basic volcanic rocks. They are usually colourless or white and are relatively soft. Formerly used as water softeners, they are used as molecular sieves in the petroleum industry and as drying agents.
Research Zeolites
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The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
©1993 - 2009 The Probert Encyclopaedia
Southampton, United Kingdom
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