Allactite

Optical character
Allactite is brownish red, optically biaxial and negative, 2V is very small, and r > v (very strong); a = 1.761, b = 1.786, g = 1.787, all ±0.003 (Larsen).

Occurrence
Allactite, a rare manganese arsenate, was formerly known only from Langban and Nordmark, Sweden. It was identified on a specimen from the Palmer shaft in the Hancock collection, a small piece of coarse brownish calcite containing grains of franklinite and a little galena. A cavity an inch across is lined with rough crystals of calcite and a few of franklinite, the allactite crystals being implanted upon both.

Figure 191
Crystal of allactite showing the forms a(100), f(210), h(101), and d(131). Palmer shaft.

They are grouped in rosettes, the largest of which is a sixth of an inch across. The crystals are attached by their orthopinacoids in subparallel groups, their terminal pyramid faces producing a gently warped surface. Minute crystals are sufficiently distinct for measurement, and there is no doubt of the identity of the species, which has been verified by blowpipe tests. The habit of the groups and oven the dominant form of the crystals are the same as those of the type material from Sweden as described by Sjogren.

Allactite has been identified in several other specimens found at Franklin, as crusts of brownish needles on the surfaces of slickensides and as very thin veins. The well-defined optical properties enable the determination of the species with certainty.

A few clusters of pale-brown crystals of allactite were found at Sterling Hill in 1929, in cavities in a calcite vein. With it were beautiful clear-green crystals of willemite, pale-blue fluorite, and a white fibrous mineral packed about the willemite.


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