1894.] Mineralogy. ~ 871 
an angle of 90° about the `c axis has taken place. The ortho-pina- 
coid has become the clino-pinacoid and vice-versa, The twinning 
plane of twinned crystals has undergone the same revolution. By 
heating crystals beyond-the temperature required for producing the 
first meta-scolecite, the double refraction of the substance steadily de- 
creases and the symmetry approaches more and more closely to the 
orthorhombic. Below red heat the structure breaks down. As scolecite 
possesses three molecules of water of crystallization, Rinne suggests 
that the first meta-scolecite contains two, the second one molecule of 
erystal water, the crystal structure being lost when all the water has 
been removed. 
Crystallization of Herderite.—Penfield' has made a study of 
herderite from the known localities as well as from a newly discovered 
locality at Paris, Me. The herderite from the latter locality as well 
as that from Hebron, contains scarcely any fluorine, its place being 
taken by hydroxyl, and the author proposes for it the name bydro- 
herderite. As the Stoneham herderite contains hydroxyl and fluorine 
in the proportions of 3:2, the one apparently replacing the other iso- 
morphieally, the name hydro-fluor-herderite is proposed for such inter- 
mediate varieties between theoretical fluor-herderite and hydro-herde- 
rite. In the crystallographic study the fact is brought out that the 
mineral is monoclinic instead of orthorhombic as has been supposed. 
This is proven not alone on Paris specimens but on specimens from 
the other localities, which were reexamined for this purpose. The 
crystals, however, approach closely to the orthorhombic system, the 
hydro- fluor-herderite being more nearly orthorhombic than the hydo- 
herderite, the substitution of fluorine for hydroxyl tending to increase 
the crystallographical axial angle and to shorten the clino-diagonal. 
It likewise diminishes the mean index of refraction and the optical 
angle. 
Composition and Related Physical Properties of Topaz.— 
Jannatsch and Locke? have shown that topaz contains water of consti- 
tution, from a chemical study of specimens from San Louis Potosi, 
Ilmen Mts., Schneckenstein, and Brazil. Penfield and Minor’ have 
independently established the same fact by a larger number of analyses, 
and shown how this greatly simplifies the formula of the mineral on 
* Am. Jour. Sci., (3) xlvii, pp. 329-339. 
5Am. Jour. Sci., (3) xlvii, pp. 386-387. 
*Ibidem, pp. 387-396. 
