Prehnite

minerals

datolite

pectolite

zeolite

Images
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 (DHZ 3 p265).

Localities

At the Jeffrey mine, Asbestos, Les Sources RCM, Estrie, Québec, Canada, prehnite occurs with grossular and diopside (R&M 94.5.442).

At Malad, Mumbai district, Maharashtra, India, prehnite pseudomorphs after laumontite have been found (KL p243).

At Calvinia, near Capetown, Cape province, South Africa, prehnite pseudomorphs after calcite have been found (KL p240).

At Croft Quarry, Croft, Blaby, Leicestershire, England, UK, prehnite occurs as disseminations and masses to 30 mm in diameter associated principally with analcime (the deposition of which it precedes) and minor calcite. The prehnite - pumpellyite wallrock alteration haloes that surround the zeolite - containing veins in the quartz - diorite contain intergrown radiating sheaves of prehnite and Fe-bearing pumpellyite together with chlorite, quartz and analcime, and also minor muscovite variety sericite, kaolinite and smectite (JRS 20.23).

At Haystack Butte, Salt River Mining District, Gila county, Arizona, USA, an intrusive event occurred over a million years ago that manifested as dolerite sills and dikes composed of anorthite and diopside. Research indicates a continuous sequence of dolerite compositions from those with fresh primary mineral assemblages and little deuteric alteration from late-stage magmatic fluids, to those that have completely recrystallised under deuteric conditions to a mixture of albite, calcic pyroxene and chlorite with accessory titanite, ilmenite, apatite and prehnite. Cores of anorthite variety labradorite laths commonly were converted to fine-grained aggregates of muscovite variety sericite, clay minerals and chlorite, and in places were converted to prehnite. This alteration happened below 440°C and at less than 1 kilobar pressure, typical conditions for the formation of prehnite. Few other species appear to be associated with the deposit. Raman analysis confirmed actinolite and tremolite on several specimens, but somewhat surprisingly there is limited evidence of epidote (R&M 95.5.432-439).

At the Upper New Street quarry, Paterson, Passaic county, New Jersey, USA, prehnite pseudomorphs after anhydrite have been found (KL p241).

At the Fanwood quarry, Union county, New Jersey, USA, prehnite pseudomorphs after glauberite have been found (KL p242).

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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