KRAISSLITE

Fe³⁺₂Mg₄Mn₄₄Zn₆(AsO₄)₆(AsO₃)₄(SiO₄)₁₂(OH)₃₆ 
Hexagonal, P6₃22, a = 8.22, c = 43.88 Å, Z = 1

	Figure 18-39. Platy, hexagonal crystals of kraisslite from Sterling Hill. Field of view is 0.6 mm in maximum dimension.

Kraisslite was first described by Moore and Ito (1978) as a platy arsenosilicate from Sterling Hill; it has not been found at Franklin. Kraisslite was redefined by Dunn and Nelen (1980), who presented the extant formula. Although its crystal structure has not been determined, it is clearly related to those of mcgovernite, hematolite, and its unnamed Fe³⁺ analogue (Dunn and Peacor (1983b).

Description

Kraisslite occurs as hexagonal, platy crystals (Figures 18-39 and 18-40) composed of only the pinacoid and prism; such neat crystals are rare. The preponderance of kraisslite exists as thin films and coatings of crystal aggregates on fracture surfaces; such coatings can be extensive. Much kraisslite occurs as thin foliae, commonly warped, with a reddish-brown to bronze-brown color. The cleavage is perfect on {0001}; the hardness is between 3 and 4; and the luster is vitreous to submetallic. The density is 3.876 g/cm³ (Moore and Ito, 1978), compared with the calculated value of 3.92 g/cm³ for the formula proposed by Dunn and Nelen (1980). Optically, kraisslite is uniaxial, positive, with w= 1.805, and weak birefringence (Moore and Ito, 1978). There is no discernible fluorescence in ultraviolet.   

Kraisslite can be confused with mcgovernite, but they are distinguished by their indices of refraction. Another similar phase is the unnamed Fe³⁺ analogue of hematolite. In both cases, X-ray powder diffraction techniques are recommended for unambiguous identification.

Composition

Kraisslite is a ferric-iron magnesium manganese zinc arsenite arsenate silicate hydroxide mineral. The formula first proposed, (Mn_0.89Mg_0.08Fe²⁺_0.03)₂₄Zn₄(AsO₄)₄(SiO₄)₈(OH)₁₂, was shown by Dunn and Nelen (1980) to be incorrect.

They proposed Fe³⁺₂Mg₄Mn₄₄Zn₆(AsO₃)₄ (AsO₄)₆ (SiO₄)₁₂(OH)₃₆ as an interim formula pending structural investigation. Kraisslite is relatively invariant in chemical composition from specimen-to-specimen, indicating that the diverse array of cations is indicative of the true composition and not the effects of random solid solution. Several representative analyses are presented in Table 15.

Occurrence and paragenesis

Kraisslite occurs, for the most part, as thin films and lenses along fractures in the ore in the central zincite zone at Sterling Hill. Veins and lenses of kraisslite have been found up to l inch thick. It is not a rare mineral; much material has been found and preserved. Kraisslite is found associated consistently with zincite and also with a number of arsenate species, such as holdenite, chlorophoenicite, sarkinite, eveite, allactite, retzian-(Nd), and others. Indeed, its occurrence with a broad array of secondary minerals indicates it might be a stable entity over a broad range of conditions.

	Figure 18-40. Slightly curved and fragmented crystals of kraisslite from Sterling Hill. Field of view is 0.5 mm in maximum dimension.

One occurrence of kraisslite warrants special mention because it occurred within the ore and not as secondary coatings. This occurrence was on the 700 level and consisted of massive segregations of kraisslite (up to 1.0 cm) within low-grade willemite-calcite- franklinite  ore. Volumetrically, kraisslite may be the most abundant arsenate mineral at Sterling Hill, but it is apparently localized in occurrence, being found only below the 500 level and never in the black-willemite ore (Parker and Troy, 1982).

Name

Kraisslite was named in honor of Alice and Frederick Kraissl, mineral collectors, both of whom contributed substantially to the growth and development of the mineral heritage of Franklin and Sterling Hill. They were strong supporters of the Franklin Mineral Museum and the Franklin-Ogdensburg Mineralogical Society. Their influence was positive and pervasive.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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This page created: January 14, 2001