Hendricksite is a potassium zinc aluminum silicate hydroxide mineral of the mica group; it is a zinc-dominant mica. There is much solid solution of Fe, Mn, and Mg, with solid solution perhaps complete to phlogopite and partially so to biotite. In general, Franklin material is more Zn-rich that that from Sterling Hill, which contains more Mg and Fe. At Franklin, Zn is always in excess of Mn. Frondel and Ito (1966) reported an unsuccessful search for mica with Mn > Zn. Hendricksite is also a common host for barium.
Hendricksite, a Zn-trioctahedral mica, was first described from Franklin by Frondel and Ito (1966a); the unit-cell parameters given above are from this study. Evans and Strens (1966) called it a zinc mica. It was subsequently reported from Sterling Hill by Frondel and Einaudi (1968) and Craig et al. (1985). This is likely the same material called biotite by Nason in Chester (1894) and by Palache (1935) and Heilwell (1986). The mineral called manganophyllite (Chester, 1894) is likely hendricksite in part. Hendricksite may have been the precursor for the local grossular-after-mica replacement-mixture locally called caswellite; see discussion under grossular and phlogopite.
The crystal structure of hendricksite was determined by Robert and Gasperin (1985) on material with 0.54 Zn per 4.00 (Si,Al). They found Zn to be exclusively in octahedral coordination and randomly distributed; there is no evidence of ordering of Zn.
Hendricksite is brown, dark brown, dark reddish-brown, and reddish-black. It is found uncommonly in euhedral crystals, but rock-locked, deformed material predominates. Frondel and Ito (1966) reported the 1M polytype to be dominant, with 2M1 and 3T polytypes also present, but only in a few specimens. The luster is vitreous and cleavage is perfect on {001}; the density is 3.40 to 3.41 g/cm3, but can vary substantially with composition. Compared with phlogopite and muscovite, hendricksite is less elastic and more easily deformed; compared with phlogopite and biotite, it is more readily soluble in acid (Frondel and Ito, 1966). There is no discernible fluorescence in ultraviolet.
The extant knowledge on the occurrence of hendricksite is drawn entirely from Frondel and Ito (1966) and Frondel and Einaudi (1968); the paragenesis has not been investigated by [Dunn]. In general, hendricksite was limited to the calcium-silicate units of the orebody at Franklin and did not occur in the common willemite-franklinite ore. Hendricksite invariably occurs with andradite, calcite, and franklinite as associated minerals. Additional associated minerals are rhodonite, feldspars, vesuvianite, bustamite, manganaxinite, and others. Frondel (1970) noted the anomalously low Sc content of Franklin hendricksite and also noted that hendricksite was generally absent from assemblages which have large amounts of pyroxene or amphibole.
At Sterling Hill, hendricksite is less abundant, due in part to a locally higher concentration of Mg and Fe in silicates, relative to Franklin. Frondel and Einaudi (1968) reported hendricksite occurring on one specimen with andradite, rhodonite, franklinite, and calcite, and on another with calcite and franklinite. (Dunn, 1995)

 Location Found: Franklin (Type Locality) and Ogdensburg, unique to Franklin/Ogdensburg area
 Year Discovered: 1966
 Formula: K(Zn,Mg,Mn2+)3(AlSi3O10)(OH)2
 Essential Elements: Aluminum, Hydrogen, Oxygen, Potassium, Silicon, Zinc
 All Elements in Formula: Aluminum, Hydrogen, Magnesium, Manganese, Oxygen, Potassium, Silicon, Zinc
 IMA Status: Approved
 To find out more about this mineral at minDat's website, follow this link   Hendricksite

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.464

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.61

The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 9, No. 1 - February 1968, pg. 13The Exclusive Minerals of Franklin/Ogdensburg, N.J. (as of January 1968) by Frank Z. Edwards - Hendricksite (Short Note)
View IssueV. 8, No. 2 - August 1967, pg. 5Mineralogical Data - Hendricksite
View IssueV. 8, No. 1 - February 1967, pg. 8Hendricksite/Manganophyllite

Hendricksite mica and andradite garnet from Franklin, NJ.
Hendricksite mica (dark brown, black) and andradite garnet (brown) from Franklin, NJ. Photo By WP.

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