16 
CORUNDUM, ITS OCCURRENCE AND DISTRIBUTION. 
Fig. 8. — Pyi-amidal crystal 
of Ceylon corundum. 
becoiul order a^ fig. 5. These hexag'oiial markings vary considerably 
in size, distinctness, regularity, etc. This zonal structure has been 
observed on many varieties of corundum. 
Sometimes the base is divided into sections 
by lines radiating from a point at the center 
to the edges of base c on the prism c/, fig. 5. 
TWINNING. 
Penetration twins of corundum have been 
observed, but they are not at all common. 
The most connnon twinning plane of corun- 
dum is parallel to the rhombohedron /' ( lOll ) > 
The most prominent twinniiig is pol3^syn- 
thetic; that is, the twinning has been re- 
peated, forming a series of plates or lamella* 
in twin position. This type of twinning often produces a laminated 
structure, and sometimes it gives rise to a surface that 
is distinctly striated, consisting of minute reentrant and 
salient angles. This twinning also produces the rhom- 
bohedral parting described below. 
Recently" a new twinning plane has been identified 
in corundum which is parallel to the base c (0001 ) . The 
crystals in which this twinning plane is developed were 
gray and ruby red in color, and have been found spar- 
ingly in the Ruby mine on Caler Fork of Cowee Creek, 
Macon County, N. C. They Avere first observed by 
Mr. W. E. Hidden, and, as stated by him, the two best 
crystals measured 6 nun. in diameter and length. They 
are characterized by reentrant angles (w, i22i3) on the 
prismatic faces (a, 1120) and some slight natural cor- 
rosion. 
PARTING. 
Fig. 4. — Corun- 
dum crystal, 
India (after 
Mallet). 
a 
Fig. 5. — Corundum crystal illustrat- 
ing radial lines in a l)asal plane 
extending from center of base to 
edges of prism. 
What Avas considered by the earlier 
mineralogists to be a cleavage in corun- 
dum has been shown to be a parting 
produced by- the action of mechanical 
or chemical forces or, perhaps, of both 
acting together. 
As early as 1802 Count de Bournon ^ 
had pointed out that certain corun- 
dums, especially sapphires, did not 
" Hidden, William E., Am. .Tour. Sci., vol. 13, 1902, p. 474. 
^ Description of the corundum stone and its varieties : Philos. Trans. Royal Soc, Lon- 
don, vol. 92, 1802, pp. 233-326. 
