THE MINERAL
PREHNITE
  • Chemistry: Ca2 Al2 Si3 O10(OH)2, Calcium Aluminum Silicate Hydroxide.
  • Class: Silicates
  • Subclass: Phyllosilicates
  • Uses: As mineral specimens and limited use for ornamental stone purposes.
  • Specimens
Prehnite was named after its discoverer; Colonel Hendrik von Prehn and is an attractive collection mineral that is occassionally used for ornamental stone purposes. Its color is usually a pleasant green and is at times quite unique to prehnite. Typical prehnite forms rather thick crusts with a rough or crystaline texture. Epimorphs (crystal growth over the surface of another mineral) over laumontite crystals are interesting and attractive. Usually the laumontite has dissolved away leaving the hollow crust of prehnite behind.
- PREHNITE EPIMORPHS AFTER LAUMONTITE

Prehnite is often found with zeolites and is sometimes thought of as a zeolite. But zeolites are actually tectosilicates and prehnite is a member the Phyllosilicates Subclass. However, like zeolites, prehnite can give off water when heated, but can not gain the water back like they can. Like most zeolites, prehnite is formed as a result of low grade metamorphism usually from hydrothermal solutions. Crystals can be found in cavities of mafic igneous rocks.

Minerals that can be confused with prehnite include gyrolite, smithsonite and hemimorphite. Prehnite is harder than all of these and lacks smithsonite's unusual luster. Hemimorphite is usually blue and gyrolite is not as glassy. Prehnite can be associated with many beautiful minerals and can make a fine specimen on its own. Its color, luster, associations and crystal habits make it a joy to own.

PHYSICAL CHARACTERISTICS:

  • Color is usually a pale green to a yellowish grass green, also gray, white or colorless.
  • Luster is vitreous to waxy or pearly.
  • Transparency: Crystals are transparent to mostly translucent.
  • Crystal System is orthorhombic; 2 m m.
  • Crystal Habits include nodular, concretionary, radial, encrusting and stalactitic formations among other similar types. Tabular or pyramidal individual crystals are rare but some nodular specimens show tabular crystal protrusions. Epimorphs (crystal growth over the surface of another mineral) over laumontite are usual, but available (see above).
  • Cleavage is good in one direction (pinacoidal).
  • Fracture is uneven.
  • Hardness is 6 - 6.5.
  • Specific Gravity is approximately 2.9+ (average).
  • Streak is white.
  • Other Characteristics: Lacks the luster of smithsonite and cleavage surfaces are curved and pearly.
  • Associated Minerals include datolite, gyrolite, fluorapophyllite, quartz, calcite, copper, pectolite, stilbite and other zeolites.
  • Notable Occurrences include the type locality of Cape of Good Hope, South Africa as well as Connecticut; Pennsylvania; Patterson, New Jersey and Centreville, Virginia, USA; Bombay, India; Harz Mountains, Germany; Austria; Scotland; Copper Valley, Namibia; Jeffery Quarry, Asbestos, Canada; China; New South Wales, Australia and France.
  • Best Field Indicators are crystal habit, color, cleavage, hardness and associations.
PREHNITE specimens:
(hover for more info)
PREHNITE specimen pre-1
$ 30.00
Dims: 4-5/8" x 2-1/2" x 2-1/4"
Wt: 11.0 oz
Malad(near Bombay), Maharashtra State, India
with Prehnite! The Prehnite has the obligatory pale green, color, waxy luster and translucence of its kind. The calcite is just plain old white calcite where visible, and rests on a basalt bed. I like it-- it almost has the luster of Smithsonite!
no photo
pre-1 ($ 30.00)
Malad(near Bombay), Maharashtra State, India
PREHNITE specimen pre-2
$ 175.00
Dims: 5.0" x 3.9" x 1.3" (12.7 x 9.9 x 3.3 cm)
Wt: 12.6 oz. (356 g)
Lane Quarry, Westfield, Massachusetts, U.S.A.
This specimen's locality isn't the only unusual aspect that it has. It is made up of a crust of intergrown Prehnite crystals with excellent orthorombic tabular form- they are easily the largest Prehnite crystals that I have ever seen! Most of them measure about 0.1 x 0.1 x 0.1" (3 x 3 x 3 mm). Many are loosely aggregated into rounded "wheels", but just as many appear to be individual. The specimen is in generally good condition with moderate damage. The Prehnites have a pale green color with a hint of yellow, a vitreous to pearly luster, and are translucent. There is another material on the underside of the specimen that I cannot identify. It is heavily weathered and may be Prehnite in a different form. It is a very impressive specimen.
no photo
pre-2 ($175.00)
Lane Quarry, Westfield, Massachusetts, U.S.A.
PREHNITE specimen pre-3
$ 65.00
Dims: 3.7" x 3.0" x 1.3" (9.4 x 7.6 x 3.3 cm)
Wt: 3.53 oz. (356 g)
Poona, India
A substantial section of Prehnite crust constitutes this specimen. There is a thin layer on its underside that could be composed of any one of several different zeolites- I think that this thin layer is made of natrolite, though. The bulk of the crust, however, is made up of Prehnite that has a very pale green coloration, a pearly luster, and excellent translucence. It has definitely botryoidal tendencies that include some bizarre formations that rise out of the crust. These formations resemble rounded stalactites that rise up from the crust at an angle. A few appear to have broken ends. The top of the crust has a very thin dusting of more natrolite, consisting of both individual crystals and botryoidal aggregates of the white mineral.
no photo
pre-3 ($ 65.00)
Poona, India
PREHNITE specimen pre-4
$ 32.00
Dims: 2.4 x 1.9 x 0.9" (6.1 x 4.8 x 2.3 cm)
Wt: 2.01 oz. (57.1 g)
O & G Quarry, Woodbury, Connecticut, U.S.A.
This small hand specimen consists of a botryoidal crust of intergrown Prehnite nodules. These nodules are not very well-formed and do show some noticeable damage. They have a pale, grayish-green coloration and a rather dull waxy luster, and range from translucent to dimly transparent, depending on the thickness of the crust in any given area. Their broken edges display the compact, fibrous and radiating habit of this material, and one nodule creates a hollow between itself and a small amount of what appears to be the host rock. There are several tiny, transparent calcites resting on this little bit of material. I think the most interesting aspect of this piece is its locality, which is an almost unknown quarry in my home state of Connecticut.
no photo
pre-4 ($ 32.00)
O & G Quarry, Woodbury, Connecticut, U.S.A.
PREHNITE specimen pre-5
$ 36.00
Dims: 3.7 x 2.3 x 1.8" (9.4 x 5.8 x 4.6 cm)
Wt: 9.4 oz. (267 g)
O & G Quarry, Woodbury, Connecticut, U.S.A.
Many rounded Prehnite nodules are intergrown to form a reniform crust on the basalt base rock of this specimen. These nodules do not exceed 0.4" (1.0 cm) in diameter, and most are in good condition, though there is some noticeable damage to a few of them. All are incomplete due to intergrowth with the basalt base or each other, and their forms are good, though not close to being perfectly spherical. All are nearly colorless, with a faint green tinge and a cloudiness that may be caused by their dull, waxy luster, and are dimly transparent. Accompanying the Prehnites are two almost cubic apophyllite crystals that are colorless, transparent, and have a good form and a bright pearly luster.
no photo
pre-5 ($ 36.00)
O & G Quarry, Woodbury, Connecticut, U.S.A.
PREHNITE specimen pre-6
$ 40.00
Dims: 1.3 x 1.2 x 1.0" (3.3 x 3.0 x 2.5 cm)
Wt: 1.52 oz. (43.1 g) w/ base
Jeffrey Mine, Asbestos, Quebec, Canada
Though they are warped, these are some of the most well-formed Prehnite crystals that I have seen. They are heavily intergrown and rest on a base made up of what appear to be dozens of tiny, intergrown grossular garnets. The orthorhombic prismatic form of the Prehnite is nearly impossible to define- though most of their edges are straight, they are also highly disseminated. Their faces are often curved or very subtly angled from each other as to give the illusion of a curve. All have a uniformly pale rusty orange coloration and a dull waxy luster that appears to be caused by a thin coating of a material that is possibly iron-bearing. A few grossular garnets that rested adjacent to the edge of the Prehnite cluster have been dislodged, allowing one to see some of the material inside- it appears to be colorless, transparent, and at least moderately clear. The host rock to which they are attached is made up of dozens of grossular garnets that do not exceed 0.1" (3 mm) in diameter. Many of them are broken, but those that are intact are in good condition, showing excellent dodecahedral form. All have a very subtle, pale pink coloration and are transparent and very clear. The specimen is hot-glued onto an acrylic base.
no photo
pre-6 ($ 40.00)
Jeffrey Mine, Asbestos, Quebec, Canada
PREHNITE specimen pre-7
$ 50.00
Dims: 3.6 x 3.6 x 2.4" (9.1 x 9.1 x 6.1 cm)
Wt: 5.06 oz (143.5 g)
Thana, India
At least 10 Prehnite epimorphs are clustered together to form this specimen. These epimorphs started out as Prehnite crusts which partly coated a cluster of laumontite crystals, which later mouldered away, leaving "molds" made of the Prehnite. The specimen is generally in good condition, as there is some noticeable damage present, but most of the Prehnite is in excellent condition. Most of it is in the form of compact, fibrous crusts that radiate outwards from their "centers", where the laumontite used to be. It has the standard pale green coloration and a waxy-to-pearly luster, and is obviously translucent. A few very small patches of broken laumontite remain, and several white, pseudocubic apophyllites have grown off of the epimorphs. There is no basalt host rock present, though.
no photo
pre-7 ($ 50.00)
Thana, India
PREHNITE specimen pre-8
$ 25.00
Dims: 2.7 x 1.1 x 0.6" (6.9 x 2.8 x 1.5 cm)
Wt: 9.8 g
Bombay, India
This small hand specimen consists of a single Prehnite epimorph- a layer of Prehnite formed around a laumontite crystal that since mouldered away. The crust is made up of hundreds of tiny, heavily-intergrown nodules that have a milky-white color with a faint green tinge and a dull, pearly-to-waxy luster. It is translucent and forms a very good cast of a laumontite crystal. A few pseudocubic apophyllites are attached to it, but there is no base or host rock present.
no photo
pre-8 ($ 25.00)
Bombay, India
PREHNITE specimen pre-10
$ 39.00
Dims:0.9x0.7x0.7" (2.3x1.8x1.8 cm)
Wt: 0.3oz. (7g)
Jeffrey Mine, Asbestos, Quebec, Canada
This is an excellent thumbnail specimen of prehnite. There are three prehnite crystals on this specimen. These prehnite crystals reach 0.3" (0.8cm) in length, and are attractively grouped on a matrix of grossular garnet crystals. The grossular garnets are of different colors, making an interesting contrast. One of the prehnite crystals is damaged on one edge; otherwise, this is a premium specimen. There is a small amount of glue on the bottom of this specimen where it was once glued to a base.
no photo
pre-10 ($ 39.00)
Jeffrey Mine, Asbestos, Quebec, Canada
PREHNITE specimen pre-11
$ 75.00
Dims:4.0x3.2x2.0" (10.1x8.1x5.1 cm)
Wt: 14.6oz. (413g)
Southbury, Connecticut
The top of this specimen is covered with a crust of botryoidal prehnite. These prehnites show a greasy to pearly luster and are somewhat translucent. The color is a pale green. This specimen shows moderate damage around the edges where it was broken from the host rock. On the other hand, the damaged nodules on the edges allow one to view the radiating structure of these nodules.
no photo
pre-11 ($ 75.00)
Southbury, Connecticut
PREHNITE specimen pre-12
$ 39.00
Dims:1.4x1.0x0.7" (3.6x2.5x1.8 cm)
Wt: 0.7oz. (19g)w/base
Jeffrey Mine, Asbestos, Quebec, Canada
This specimen consists of several intergrown, elongated crystals of prehnite on a bit of matrix material. The longest crystal appears to be about 1.0" (2.5cm) in length. Some of the crystals on this specimen are double terminated. There is no damage to this specimen. This is simply a wonderful thumbnail specimen from a classic location. This specimen is firmly glued to a clear acrylic base.
no photo
pre-12 ($ 39.00)
Jeffrey Mine, Asbestos, Quebec, Canada
PREHNITE specimen pre-13
$ 54.00
Dims:1.5x1.0x0.7" (3.8x2.5x1.8 cm)
Wt: 0.6oz. (17g)w/base
Jeffrey Quarry, Asbestos, Quebec, Canada
This is yet another excellent specimen from the Jeffrey Quarry. A crust of wonderfully euhedral prehnite crystals (to 0.3", 0.8cm) rests on a bit of matrix with minor diopside. These prehnites show great crystal form, and good translucency. There is no damage to this specimen. It is firmly glued to a clear acrylic base.
no photo
pre-13 ($ 54.00)
Jeffrey Quarry, Asbestos, Quebec, Canada
PREHNITE specimen pre-14
$ 53.00
Dims: 3.29x2.56x1.80" (8.35x6.51x4.57cm)
Wt: 10.74oz (304g)
Wave Hill, Northern territory, Australia
This is a chunk of weathered prehnite, cut and polished on one surface to reveal the crystals within. The unpolished surface is rounded, weathered, and brown, showing no crystal indications. The polished surface, however, reveals that this is essentially a single large crystal (with multiple fractures). It is translucent but nearly transparent, and a pretty yet pale green. The polished surface is vitreous. One end of the specimen does exhibit a very interesting growth pattern, especially when using a loupe.
no photo
pre-14 ($ 53.00)
Wave Hill, Northern territory, Australia
PREHNITE specimen pre-15
$ 32.00
Dims mm=65.79x56.46x35.56
wt g=195.7
Wave Hill, Northern Territory, Australia
This weathered brown nodule has been cut and polished on one face to reveal the transparent pale green prehnite at its core.
no photo
pre-15 ($ 32.00)
Wave Hill, Northern Territory, Australia
 

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