JOHNBAUMITE

Ca₅(AsO₄)₃(OH) 
Hexagonal, P6₃/m, or P6₃, a = 9.70, c = 6.93 Å, Z = 2

Johnbaumite was described from Franklin by Dunn et al., (1980a) and from Sterling Hill by Jenkins (1994).

Description

Johnbaumite is known primarily as massive material, gray-white, with a greasy to subadamantine luster. Small crystals were reported by Jenkins (1994). The hardness is approximately 4½, and cleavage is distinct on {0001}. The density is 3.68 g/cm³ (meas.) 3.73 g/cm³ (calc.). Optically, it is uniaxial, negative, with w = 1.687 and e = 1.684.

Johnbaumite fluoresces with an orange color in shortwave ultraviolet, but this is not diagnostic. It is easily confused with other arsenic apatites and is best verified by a quantitative water determination, coupled with X-ray methods.

Composition

Johnbaumite is a calcium arsenate hydroxide mineral of the apatite group and the hydroxyl analogue of turneaureite. A microprobe analysis is given in Table 24. True svabite is unknown locally. The analysis given by Palache calculates as a poor one but, if reasonable, is of what is now known as johnbaumite; the same is true for that given by Bauer and Berman (1930).  

Occurrence and paragenesis

Johnbaumite was found in a small lenticular body located at 830S, 170E, in the 730S Palmer Shaft pillar, in the third sub-level below the 750 level in the Franklin Mine in 1944. Johnbaumite is host for an occurrence of Franklin yeatmanite (Dunn and Leavens, 1980) and is also associated with diopside, andradite, copper, franklinite, and romeite.

Prismatic, gray crystals, over 4 cm in length and approximately 1 cm wide, were found on the 1200 level at Sterling Hill, associated with white calcite, barite, sonolite, zincite, and adelite, and are best referred to as johnbaumite-svabite. They are both F and (OH)-bearing, but their F/(OH) ratio is uncertain; preliminary studies suggest they have (OH) > F. However, they are rimmed with and contain inclusions of minute crystals of adelite; bulk water determinations are therefore suspect. Another probable Sterling Hill occurrence was described by Jenkins (1994).

In general, Sterling Hill apatites are Ca-AsO₄ members of the group and, because fluorine is less abundant here than at Franklin and (OH)-bearing minerals are common, johnbaumite may be locally abundant.

Name

Johnbaumite is named in honor of John L. Baum, former resident geologist at Franklin, curator of the Franklin Mineral Museum, and a strong contributor to the whole of the Franklin mineral culture. He is one of very few Franklin Renaissance men, and his contributions are discussed further in the section entitled “Men of distinction.”

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