HENDRICKSITE-1M

K(Zn,Mg,Mn)₃AlSi₃O₁₀(OH)₂
Monoclinic, C2/m, a = 5.37, b = 9.32, c = 10.30 Å, b = 99^o, Z = 2

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.

	Figure 18-1. Altered platy mica (reflective white) (caswellite) with andradite (dark gray) from Franklin. Granular franklinite-willemite ore is at bottom right. Specimen is 10 cm in maximum dimension. Privately owned. Photo by the author.

Hendricksite may have been the precursor for the local grossular-after- mica replacement-mixture locally called caswellite (Figure 18-1); see discussion under grossular and phlogopite.

Crystal structure

The crystal structure of hendricksite was determined by Robert and Gaspérin (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.

Description

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). Optically, hendricksite is biaxial, negative, 2V = 8^o, with a = 1.598, b = 1.658, and g = 1.660 for crystals with 40 mole % of the hendricksite end-member (Evans and Strens, 1966). There is no discernible fluorescence in ultraviolet.

Composition

Hendricksite is a potassium zinc aluminum silicate hydroxide mineral of the mica group; it is a zinc-dominant mica. Representative analyses are given in Table 13 and show part of the range of composition. 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.

Occurrence and paragenesis

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 the writer. 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.

Name

Hendricksite was named for Dr. Sterling B. Hendricks (1902-1981), an American crystallographer who established the polymorphs of the micas.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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