Prehnite

phyllosilicate

zeolite

Formula: Ca2Al(Si3Al)O10(OH)2
Phyllosilicate (sheet silicate)
Specific gravity: 2.8 to 3.0
Hardness: 5½ - 6
Streak: White
Colour: Colourless, white, grey, greenish, yellowish, reddish
Solubility: Slightly soluble in hydrochloric acid
Environments:

Plutonic igneous environments
Metamorphic environments
Hydrothermal environments
Basaltic cavities (most common)

Prehnite occurs as a secondary mineral lining cavities in basalt and related rocks, and in alpine crevices. It is associated with zeolites, datolite, pectolite and calcite. Prehnite is characteristic of the prehnite-pumpellyite facies, and it is also a mineral of the zeolite facies.
In metamorphic rocks prehnite is found in contact-altered limestone and marl.

Alteration

grossular and lawsonite to zoisite, prehnite and H2O
Ca3Al2(SiO43 + 3CaAl2(Si2O7)(OH)2.H2O ⇌ 2Ca2Al3[Si2O7][SiO4]O(OH) + Ca2Al(Si3Al)O10(OH)2 +10H2O
Increasing temperature faours the forward reaction (SERC)

laumontite and calcite to prehnite, quartz, H2O and CO2
CaAl2Si4O12.4H2O + CaCO3 → Ca2Al(Si3Al)O10(OH)2 + SiO2 + 3H2O + CO2
Prehnite and pumpellyite form from the Ca zeolites in the presence of calcite, as in the above equation (DHZ 5B p127).

lawsonite, grossular and quartz to prehnite
CaAl2(Si2O7)(OH)2.H2O + Ca3Al2(SiO43 + SiO2 ⇌ 2Ca2Al(Si3Al)O10(OH)2
Increasing temperature favours the forward reaction (SERC)

prehnite to zoisite, grossular, quartz and H2O
5Ca2Al(Si3Al)O10(OH)2 ⇌ 2Ca2Al3[Si2O7][SiO4]O(OH) + 2Ca3Al2(SiO43 + 3SiO2 + 4H2O
Increasing temperature favours the forward reaction (SERC)

prehnite and lawsonite to zoisite, quartz and H2O
Ca2Al(Si3Al)O10(OH)2 + 2CaAl2(Si2O7)(OH)2.H2O ⇌ 2Ca2Al3[Si2O7][SiO4]O(OH) + SiO2 + 8H2O
Increasing temperature favours the forward reaction (SERC)

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