Prehnite

prehnite

datolite

pectolite

zeolite

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Formula: Ca2Al(Si3Al)O10(OH)2
Phyllosilicate (sheet silicate)
Crystal System: Orthorhombic
Specific gravity: 2.8 to 2.95 measured, 2.9 calculated
Hardness: 5½ to 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 the Shijiang Shan-Shalonggou mining area, Inner Mongolia, China, the mineral deposits occur predominantly in veins of hydrothermal origin in skarn. Prehnite was found associated with dark red garnet, axinite and lustrous apophyllite. The rhombohedral disc-shaped crystals fill gaps between garnet crystals and axinite; they range in size up to 8 mm (R&M 96.5.404).

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 Glen Brittle, Minginish, Isle of Skye, Eilean á Chèo, Highland, Scotland, UK, vesicles are abundant in basaltic lava, mostly filled with mordenite-quartz intergrowths. A few vesicles, up to 90 mm across, exhibit fillings dominated by coarse-grained quartz and calcite, without mordenite. In these, the outermost few millimetres of the filling comprises greyish white radiating prehnite, commonly in the form of spherules up to 3 mm across (JRS 23.86-90).

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).

The Central Mine, Central, Keweenaw county, Michigan, USA, initially targeted a series of sub-parallel mineralised fissure veins where the most copper-rich portion of the vein was close to the base of the main greenstone flow.
Along with quartz, prehnite is one of the two main gangue minerals of the Central mine fissure. The mine has produced numerous fine examples of botryoidal to crudely crystallised green prehnite. Attractive specimens of pale to lime-green crystallised prehnite associated with copper, silver, calcite, epidote and other minerals have been found here (MinRec 54.1.53-81).

At the Copper Falls Mine, Copper Falls, Keweenaw county, Michigan, USA, mineralisation occurs primarily in hydrothermal veins cutting preexisting lavas and as amygdules in the Ashbed flow.
Prehnite is one of the more important gangue minerals found in fissure veins here. It occurs as colourless, white or pale green masses in veins and as amygdule fillings in the Ashbed flow. Botryoidal pale green prehnite is not uncommon in vugs in the veins. Associated minerals include quartz, copper, silver and various zeolites (MinRec 54.1.112).

The Cliff Mine, Phoenix, Keweenaw county, Michigan, USA, is situated at the base of a roughly 70-metre basalt cliff. A curious feature of the impressive thickness of the greenstone flow here is that it contains zones of “pegmatoid”: areas where slow cooling in the core of the lava flow allowed for large feldspar crystals exceeding 1 cm to grow. Such features are normally only observed in intrusive igneous rocks and are almost unheard of in basalt flows.
The Cliff mine primarily exploited rich copper mineralisation in the Cliff fissure (vein). Although mineralised with copper to some extent along its entire length, the part of the vein just below the greenstone flow carried the richest copper mineralisation by far. A significant amount of the copper recovered at the Cliff mine came from amygdaloids in the tops of 13 basalt flows which were cut by the Cliff vein. The discovery and mining of this vein proved that the veins were the source of the large masses of float copper that were already well known, and proved that the primary ore mineral in the district was native copper, not sulphides, as had been suspected earlier.
Prehnite is one of the most common vein-filling species at the Cliff mine. White, brown or especially pale green masses and botryoidal aggregates are common, often with associated quartz, epidote and copper. The brown aggregates are often found to be coloured by minute inclusions of copper (MinRec 54.1.25-49).

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).

The Purple Diopside Mound, Rose Road, Pitcairn, St. Lawrence county, New York, USA, is situated in marble. The development of veins of large crystals probably occurred as a result of fluid penetration from a concurrent intrusion. Many of the minerals of interest to collectors formed during this primary event, with additional species resulting from the subsequent alteration of scapolite. There seems to be little, if any, secondary, late-stage mineralisation present.
Prehnite forms translucent tan botryoidal masses to 9 cm associated with pyrite and calcite. It is also rarely present as minute rounded crystals on purple diopside. The prehnite most likely formed as an alteration product of meionite (R&M 96.6.552).

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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