IDENTIFICATION OF FERTILIZER MATERIALS. 11 
Gypsum. CaS0 4 + 2H 2 0.— Monoclinic. 
Optically positive ( + ). Plane of optic axes parallel to (010) 
at ordinary temperatures. 
BxJ5 = +52^°. Dispersion inclined strong. 
2Vy = 58° 8'. 2E r = 95° 14'. Refractive indices, ar = 1.5205, 
£=1.5226, r= 1.5296. 
Limestone. Calcite. OaC0 3 . — Rhombohedral. Uniaxial. Opti- 
cally negative ( — ). Double refraction strong. Refractive indices, 
co= 1.658, £ = 1.486. Dolomite, (Ca,Mg)C0 3 , is very similar; but the 
refractive* index is greater, &>=1.68, £ = 1.50. Various chemical 
tests may be used to distinguish the two. 1 
The mineralogical composition of ground rocks and ground min- 
erals may be ascertained by petrographic methods. On account of 
the large number of minerals which might be present in such prod- 
ucts, reference is here made to the literature already cited for their 
optical properties. 
The above substances are readily differentiated under the micro- 
scope. Potassium chloride, being isometric, is isotropic, and conse- 
quently remains dark during a complete rotation of the microscope 
stage between crossed nicols. The refractive index serves as addi- 
tional evidence and distinguishes this salt from halite (refractive 
index =1.54) and other isotropic substances. The remaining sub- 
stances are divided into two groups by means of interference figures. 
The rhombohedral and hexagonal substances give uniaxial figures; 
and the orthorhombic and monoclinic substances give biaxial figures. 
The uniaxial substances are sodium nitrate, apatite, calcite, and dol- 
omite. All of them are optically negative, and consequently this 
characteristic does not aid in distinguishing them apart. The deter- 
mination of the refractive indices, however, readily distinguishes them. 
The biaxial substances are potassium sulphate, ammonium sulphate, 
kainite, calcium nitrate, and gypsum. The optical character differen- 
tiates these into the optically positive group consisting of potassium 
sulphate, ammonium sulphate, and gypsum; and the optically nega- 
tive group, kainite and calcium nitrate. In the former group potas- 
sium sulphate is immediately distinguished by its low index of refrac- 
tion. Ammonium sulphate and gypsum may be distinguished by the 
other optic constants given. In the optically negative group kainite 
and calcium nitrate may be distinguished by the difference in size of 
the optic angle. 
Phosphate rock can not be distinguished microscopically as such. 
But the mineralogic nature of the material, absence of other phos- 
phatic material, and presence of water-insoluble phosphoric acid, 
jointly, are strong indications of it. 
i Iddings, Rock Minerals, p. 34. 
