= 
= 
‘laws of thermo-chemistry.* 
1887] Mineralogy and Petrography. 375 
four illustrations of copper crystals and crystal groupings.—— 
From new measurements of crystals of hyalosiderite, the iron-rich 
olivine, and forsterite, the pure magnesian variety, Max Bauer’ 
has recalculated the axial relations of these minerals. For the 
first he finds a:b:c = .46815 : 1: .58996; for the second, a:b c = 
.46476:1:.58569. On comparing these ratios with those ob- 
tained from measurements made on fayalite, the pure iron olivine, 
and other members of this series, the composition of which is 
known, it is found that an increase in the amount of the magnesium 
molecule present in any case is accompanied by a shortening of 
the a andc axes as compared with the b axis. In the same 
article Bauer describes twinning lamellæ in massive darite from 
several localities in Germany. The twinning plane is 6Px. é 
regards them as pressure twins like those found in calcite, 
cyanite, sphene, etc. 
Miscellaneous.—In the January number of the American 
Fournal of Science? Mr. G. F. Becker has an interesting article 
ture as characteristic of metamorphosed sediments.3 Porphyrite 
i i 
magma, and is merely the evidence of fractional crystallization 
of the various minerals. These conclusions are base 
theory of the solidification of minerals in accordance with certain 
In a letter to the Neues Fahrbuch 
für Mineralogie A. Schrauf explains his views on morphotropism 
and atometrie, which, so far as they relate to minerals, are briefly 
as follows. The form of crystallization of chemical mixtures de- 
pends principally upon the various amounts of their constituents. 
MgSO,+ 7H; ( E it ) F: t l z th th x h bi J t , 
while FeSO, + 7H,O (melanterite) is monoclinic. Mixtures of 
MgSO, + 7H,O and FeSO, + 7H,0O crystallize in the latter system 
until the proportion of magnesium in the mixture is to the amount 
of iron as three atoms to one atom (3MgSo, + FeSo,) + 7H,O, 
when it crystallizes in the orthorhombic system,—z.¢., the crystal- 
lization is determined by that substance which is in excess (by 
weight). In the compound (3MgSo, + FeSo,) + 7H,O the Mg and 
1 Neues Jahrb. f. Min., 1887, i. p. 1. 
2 Amer. Jour. Sci., January, 1887, p. 50. 
3 Cf. American Naturalist, Notes, December, 1886, p. 1050. 
4 Amer. Jour. Sci., 1886, p. 120. 
5 Neues Jahrb. f. Min., 1886,.i. p. 234. 
# 
