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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).
Prehnite from the Jeffrey Mine
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).
Prehnite from Shijiangshan - Image
At Malad, Mumbai district, Maharashtra, India, prehnite pseudomorphs
after laumontite have been found
(KL p243).
Prehnite from the Malad Quarry - Image
At Calvinia, near Capetown, Cape province, South Africa, prehnite pseudomorphs
after calcite have been found
(KL p240).
Prehnite from Calvinia - Image
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).
Prehnite from Haystack Butte - Image
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).
Prehnite from the Central Mine - Image
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).
Prehnite from the Cliff Mine - Image
At the Upper New Street quarry, Paterson, Passaic county, New Jersey, USA, prehnite
pseudomorphs after anhydrite have
been found
(KL p241).
Prehnite from the Upper New Street Quarry - Image
At the Fanwood quarry, Union county, New Jersey, USA, prehnite pseudomorphs
after glauberite have been found
(KL p242).
Prehnite from the Fanwood Quarry - Image
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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