Prehnite



Prehnite is a calcium aluminum silicate hydroxide mineral. An analysis by Lawson Bauer of the radial, pink-stained aggregates yielded: SiO2 40.77, A12O3 25.46, CaO 23.08, ZnO 2.36, MgO 0.66, FeO 1.00, MnO 1.06, H2O 5.69, total = 100.08 wt. %. Bauer also analyzed the more common Franklin prehnite and found 4.11 wt. % MnO, with FeO and MgO both < 0.3 wt. %.
Prehnite was first reported from Franklin by Palache (1935); it had been studied at the end of the last century by C. H. Warren when the minerals from the Parker Dump were first encountered and a number of new species described. It has not been reported from Sterling Hill.
Franklin prehnite in its several habits has an appearance wholly atypical for the species. Here it occurs as pearly, lamellar, platy crystals and aggregates, which vary substantially in grain size. It also occurs as spherical (2-4 cm) radial sprays of prismatic crystals. It is commonly white to grayish white to colorless, but some tight radial sprays have pink stains, the two colors forming pink-and-white concentric bands. Cleavage is imperfect; the density is 2.89-2.96 g/cm3 (Palache, 1935); and the luster varies from pearly to vitreous. Some prehnite is fluorescent in ultraviolet (Bostwick, 1982) with an orange-pink color in shortwave. Local prehnite is best distinguished from similar species using X-ray and optical methods.
The main assemblage for Franklin prehnite is from the Parker Mine and consists of large massive specimens composed mostly of prehnite, pectolite, margarosanite, manganaxinite, franklinite, and andradite, with minor amounts of willemite, mica, hancockite, xonotlite, and clinohedrite.
A less abundant occurrence of Franklin prehnite is one consisting of spherules, up to 4 cm in diameter, of radiating prismatic crystals. This material closely resembles the common habit of much pectolite from zeolite assemblages elsewhere. The few specimens seen are associated with an unanalyzed brown mica. A notable feature of these spherules is concentric bright-pink bands, sometimes irregular, which are apparently the result of staining by an unknown agent. Some prehnite may be mislabeled as wollastonite or pectolite in older collections. (Dunn, 1995)


 Location Found: Franklin
     
 
 Year Discovered: 1788
     
 Formula: Ca2Al2Si3O10(OH)2
 Essential Elements: Aluminum, Calcium, Hydrogen, Oxygen, Silicon
 All Elements in Formula: Aluminum, Calcium, Hydrogen, Oxygen, Silicon
     
 IMA Status: Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"
     
Fluorescent Mineral Properties

 Shortwave UV light: Moderately bright orange to bluish orange to lavender-pink
     
 To find out more about this mineral at minDat's website, follow this link   Prehnite

     
 References:
Dunn, Pete J. (1995). Franklin and Sterling Hill New Jersey: the world's most magnificent mineral deposits. Franklin, NJ.: The Franklin-Ogdensburg Mineralogical Society. p.502

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.71


The Picking Table References
 PT Issue and PageDescription / Comment
V. 58, No. 1 - Spring 2017, pg. 16Fluorescent Minerals of Franklin and Sterling Hill, N.J., Part 2, Richard C. Bostwick - Prehnite
View IssueV. 45, No. 1 - Spring 2004, pg. 13The Art of Fluorescent Mineral Photography, With Special Attention to the Minerals of Franklin and Sterling Hill Photographing the More Popular Franklin and Sterling Hill Fluorescent Minerals - Prehnite
View IssueV. 33, No. 2 - Fall 1992, pg. 11The Check List of Franklin-Sterling Hill Fluorescent Minerals - Prehnite (Fluorescent Info)
View IssueV. 18, No. 2 - September 1977, pg. 17The Fluorescent Minerals of Franklin and Sterling Hill, NJ by Richard C. Bostwick - Prehnite
     
Images

     
Prehnite, willemite, axinite, margarosanite, xonotlite, FranklinPrehnite, willemite, axinite, margarosanite, xonotlite, Franklin under shortwave UV Light
Prehnite, willemite, axinite, margarosanite, xonotlite, includes non-fluorescent andradite garnet and hendricksite mica, from Franklin, New Jersey. From the collection of, and photo by JVF.
Prehnite, willemite, axinite, margarosanite, xonotlite, includes non-fluorescent andradite garnet and hendricksite mica, from Franklin, New Jersey, under shortwave UV light. The prehnite fluoresces yellow-orange, willemite green, axinite red, margarosanite blue and the xonotlite purple. From the collection of, and photo by JVF.


Prehnite, xonotlite, pectolite, clinohedrite, axinite-(Mn), calcite, willemite and andradite garnet from Franklin, NJPrehnite, xonotlite, pectolite, clinohedrite, axinite-(Mn), calcite, willemite and andradite garnet from Franklin, NJ under shortwave UV Light
Prehnite, xonotlite, pectolite, clinohedrite, axinite-(Mn), calcite, willemite and andradite garnet from Franklin, NJ. 2" X 1 3/8". From the collection of Alex & Gary Kerstanski, photo by Alex Kerstanski.
Prehnite, xonotlite, pectolite, clinohedrite, axinite-(Mn), calcite, willemite and andradite garnet from Franklin, NJ under shortwave UV light. The prehnite fluoresces lavender purple, xonotlite purple-blue, pectolite orange, clinohedrite brighter orange, and the calcite red-orange, axinite-(Mn) deep red, calcite red-orange and the willemite green, the andradite is non-fluorescent. 2" X 1 3/8". From the collection of Alex & Gary Kerstanski, photo by Alex Kerstanski.







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