Hancockite



Hancockite is a calcium lead aluminum silicate hydroxide mineral of the epidote group. The original data are lower in Pb and Fe and higher in Ca and Mn than those subsequently provided by Dunn (1985b). Hancockite is unique in the epidote group in containing essential lead Pb. The red color may be due to the presence of Mn3+ (Penfield and Warren (1899).
Hancockite was originally described by Penfield and Warren (1899). Paragenetical information was provided by Hurlbut and Baum (1960). Additional chemical data, obtained on single crystals, was given by Dunn (1985b).
The crystal structure of hancockite was described and the relation to epidote confirmed by Dollase (1971), who noted the diffuseness of the X-ray reflections from this species. Hancockite has Ca and (Pb,Sr) ordered in the A(1) and A(2) sites, respectively.
Hancockite occurs as superb euhedral 1-2 mm crystals in vugs, as well as massive intergrowths with other species; the latter are much more abundant. Euhedral crystals are bright and sharp. Hancockite is invariably red, grading to reddish brown, pinkish brown, and brown when mixed with other species; the luster is vitreous to dull; and the density is 4.03 g/cm3. There is no discernible fluorescence in ultraviolet.
Hancockite was first found on the Parker Dump; later, in 1923, some dumps were partly removed for reprocessing, and it was then found in quantity. The best hancockite crystals occur lining vugs in recrystallized assemblages composed largely of andradite, manganaxinite, mica, willemite, and franklinite, together with other locally rare species, such as xonotlite. Additional associated minerals were reported by Penfield and Warren (1899) to be copper, lead, clinohedrite, vesuvianite, datolite, and barite. Fine samples are prized.
The bulk of the preserved hancockite is massive brick-red dull-lustered material, mixed with shards and remnants of the above-listed species. This material, found in large amounts, is very inhomogeneous at the microprobe level and has not been extensively investigated. Hancockite also occurs disseminated in feldspar. Frondel (1972) reported hancockite replacing manganaxinite, a relation not investigated by [Dunn]. Some massive hancockite is severely altered; prehnite occurs in such specimens. (Dunn, 1995)


 Location Found: Franklin (Type Locality)
     
 Mineral Note: Epidote-(Pb) was the approved name for Hancockite between 2006 and 2015. In July 2015, the IMA approved a proposal by Olav Revheim and Vandall King to revert the approved name to hancockite. Non-fluorescent red-brown mineral, often associated with rare Franklin mineral species.
 Year Discovered: 1899
     
 Formula: {CaPb}{Al2(Fe3+,Mn3+)}(Si2O7)(SiO4)O(OH)
 Essential Elements: Aluminum, Calcium, Hydrogen, Iron, Lead, Oxygen, Silicon
 All Elements in Formula: Aluminum, Calcium, Hydrogen, Iron, Lead, Manganese, Oxygen, Silicon
     
 IMA Status: Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"
     
     
 To find out more about this mineral at minDat's website, follow this link   Hancockite

     
 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.405

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.59


The Picking Table References
 PT Issue and PageDescription / Comment
V. 56, No. 2 - Fall 2015, pg. 27Hancockite Reinstated as Species Name, Olav Revheim and Vandall T. King
View IssueV. 47, Combined Issue 2006, pg. 21Hancockite Renamed Epidote-Pb Earl R. Verbeek
View IssueV. 33, No. 1 - Spring 1992, pg. 20The Lead Silicate Minerals of Franklin, New Jersey: an SEM Survey, Herb Yeates, Hancockite
View IssueV. 28, No. 1 - Spring 1987, pg. 25Mineral Notes Research Reports, The Roeblingite Assemblage, Hancockite
View IssueV. 9, No. 1 - February 1968, pg. 13The Exclusive Minerals of Franklin/Ogdensburg, N.J. (as of January 1968) by Frank Z. Edwards - Hancockite
     
Images

     
Hancockite, franklinite, andradite garnet and minor willemite from Franklin, NJ.
Hancockite (red), franklinite (black), andradite garnet (brown) and minor willemite from Franklin, NJ. From the collection of, and photo by JVF.


Hancockite (brick red), franklinite, andradite garnet, willemite and minor hendricksite mica from Franklin, NJ
Hancockite (brick red), franklinite (black), andradite garnet (golden brown), willemite (gemmy aqua green) and minor hendricksite mica (black) from Franklin, NJ. 3 1/2" x 2 3/4". From the collection of, and photo by Robert A. Boymistruk.







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