1890.] Mineralogy and Petrography. 671 
as among the many lead salts produced by the decomposition of 
bournonite and galena at the Mina Beatriz, Sierra Gorda, Chili. 
Miscellaneous. —Pseudomorphs of talc after quartz, magnesite and 
calamine are found not uncommonly ina talc layer at Göpfersgrün, near 
Wunsiedel in the Fichtelgebirge, near a contact of granite and lime- 
stone, that has suffered dolomitization as the result of the action of 
magnesian solutions emanating from the eruptive rock. The genesis 
and growth of the talc pseudomorph after quartz are carefully described 
by Weinschenck as taking place in the following manner. The 
quartz crystals are first traversed by numerous capillary cracks, run- 
ning parallel to the prismatic faces and rarely parallel to the rhombo- 
hedral planes. Along the sides of these, little plates of talc are devel- 
oped. From certain points within the crystal other fissures then begin 
to form, and along the sides of these more talc is formed, until finally 
there remain only a few isolated areas in which the original mineral 
can be detected. The production of the new mineral along the 
cracks leads to the supposition that the change is due entirely to the 
medium of circulating water, a view that is substantiated by experi- 
mental researches. An important contribution to the discussion of 
optical anomalies has been made by Erb in a study of sodium acetates 
of copper, magnesium, nickel, and other metals. When allowed to 
crystallize slowly these salts form at first uniaxial crystals belonging to 
the hexagonal system. As they increase in size the crystals become 
twinned according to certain orthorhombic laws. In thin section 
they show twinning lamelle, which disappear when the temperature is 
raised to 65?. The etched figures produced in both the simple and 
twinned crystals are of the same shape. They lie in the same relative 
positions, and are not symmetrically developed about the twinning 
planes of the lamellz, but they have the symmetry belonging to the 
hexagonal system. The twinned crystals are pyroelectric, but upon 
assuming the isotropic condition they lose this property entirely. To 
account for these anomalies the author thinks that a strain has been 
superinduced in them during their growth. If they are mimetitic forms 
it is odd that the crystals should possess a certain grade of symmetry when 
small, and assume it again when heated.——Upon treating freshly 
precipitated ferric hydroxide at 250? with water, to which a trace of 
ammonium fluoride has been added, Bruhns V obtained little plates of 
hematite with hexagonal cross sections. Freshly precipitated alumina 
35 Zeits. f. Kryst., XIV., p. 305. 
16 Neues Jahrb. f. Min. etc., B. B. VL, p. 121. 
V Neues. Jahrb. f. Min., etc., 1889, I1., p. 62. 
