Distinctions between the various plagioclase species. 
The species lettered b-e are, as is well known, isomorphous mixtures of the 
two terminal members of the series — albite (ab) and anorthite (an). In physical 
and optical, as well as chemical characters, there are perfect transitions, and 
oligoclase, andesite, labradorite and bytownite are simply named members of the 
series. 
As shown by Schuster’s investigations, it is possible to distinguish the plagioclase 
species by determining the direction of extinction in cleavage (planes parallel to 
O and i-i. 
The "above given data of extinction refer to the customary position of the 
examined plagioclase, (the upper O surface inclines from left to right, as well 
as to the front,) and always to the obtuse angle O. i-i, i. e. the surface i-i lying 
to the right. The -|- sign in the case of cleavage plates || O indicates that the 
direction of extinction is inclined to the obtuse angle O : i-i in the direction of 
the right prismatic angle; in cleavage plates i| i-i that it is inclined as i-i with i-i 
the — sign in both cases indicates the opposite direction. 
C. Aggregates. 
Aggregates are never dark between crossed Nicols because the numerous 
minute crystals are irregularly distributed. If the aggregate has a radial fibrous 
structure a fixed interferance figure may appear. Very often such aggregates 
may only he determined by chemical means. 
1. Serpentme (Mgj Si2 H.2 Og aq.) Specif, gr. 2. 5-2. 7. Green, yellow or 
brown, to black. Characterized by the mesh structure resulting from decay of 
olivine. In other cases the substance consists of large plates which may be 
regularly arranged at right angles to each other. It occurs as an independent 
rock mass or a decomposition product or pseudomorph after olivine. 
2. Viridite. Decomposition products of augites and hornblendes as well as 
garnet and biotite. (Delessite, chlorophaite, etc.) 
3. Basite. Decomposition product of pyroxene, very similar to serpentine. 
4. Chalcedony. A secondary mineral except in quartz sphaerulites. 
5. Zeolites. Include natrolite, scolezite, stilbite, de.smite, and chabasite. 
6. Carbonates. Aragonite. Decomposing with effervescency in H Cl, 
Easily distinguished from calcite by crystal form. 
