GROUTITE

Mn³⁺O(OH)
Orthorhombic, Pbnm,  a = 4.568, b = 10.581, c = 2.885 Å, Z = 4

	Figure 22-57. Bladed crystals of antimonian groutite from Franklin. Field of view is 1 mm in maximum dimension.

Groutite was first reported from Franklin by Klein and Frondel (1967) and Klein (1968). Specimens were commonly mislabeled manganite, and Palache (1935), in his description of manganite, an exceedingly rare mineral locally, may have examined groutite; the matter is unresolved. Groutite has not been reported from Sterling Hill.

Description

Groutite occurs in two distinct habits. The first of these, reported by Klein and Frondel (1967), occurs as platy, irregular, bladed, opaque 0.5 mm crystals, black, with strong reddish-brown internal reflections and perfect {010} cleavage (Figure 22-57). This groutite is rare; only one specimen is known. The more common groutite is strongly prismatic in habit, opaque, and black (Figures 22-58 and 22-59). Groutite also occurs as dense mats of thin acicular reddish-brown crystals (Figure 22-60). It is best identified using X-ray diffraction techniques.

	Figure 22-58. Prismatic crystals of groutite from Franklin. Field of view is 3 mm in maximum dimension.		Figure 22-59. The terminations of several crystals from figure 22-58. Field of view is 0.1 mm in maximum dimension.

Composition 

Groutite is a manganese oxide hydroxide mineral. The material described by Klein and Frondel (1967) is antimonian; they gave the composition as Sb₂O₅ 7.6, Mn₂O₃ 80.9 wt. %. The writer’s analysis of this material yielded Mn₂O₃ 79.8, SiO₂ 0.4, Fe₂O₃ 0.5, MgO 1.4, ZnO 1.3, Sb₂O₅ 5.5, with H₂O 11.1 by difference, total = 100 wt. %. The more common prismatic groutite is Sb-free with composition Mn₂O₃ 88.6, Fe₂O₃ 0.5, MgO 0.6, ZnO 0.4, H₂O 9.9 by difference, total = 100.0 wt. %.

Occurrence

	Figure 22-60. Filamentary crystals of groutite from Franklin. Field of view is 0.5 mm in maximum dimension.

The antimonian groutite reported by Klein and Frondel (1967) occurs in an open seam in green andradite, associated with manganite, cahnite, romeite, franklinite, and a Ca-As apatite.

The groutite represented in local and systematic collections is of a distinctly different paragenesis and occurs as superb crystals (Figures 22-58 and 22-59) in vugs in a distinctly brown andradite associated with brown willemite. The vugs have formed by the dissolution of barite, leaving rectangular, relict, inherited networks of fine-grained hetaerolite. Hetaerolite is the most common mineral lining such vugs. Hausmannite, manganite, calcite, kentrolite, and cahnite are also associated minerals. Much local groutite from this assemblage has been mislabeled manganite (Dunn, 1987).

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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This page created: January 11, 2001