é 
Want of Identity between Microlite and Pyrochlore. 117 
I shall first exhibit the mineralogical differences between the 
two minerals, in order to show the fallacy of any attempt to unite 
the two species on natural-history grounds; and shall then inquire 
what support the new view acquires from chemistry. 
Pyrochlore. Microlite. 
Crystals, unmodified regular] Crystals, regular octahedron, 
octahedrons. with edges truncated and angles 
|surmounted by four-sided pyra- 
mids, whose faces correspond to 
the octahedral planes. 
_ Cleavage, none (fracture con- Cleavage, octahedral, distinct. 
choidal). 
Lustre, resinous to vitreous. Lustre, resinous. 
Color, deep red-brown to black.| Color, pale honey-yellow. Sur- 
faces of crystals often reddish or 
blackish brown from inpen 
minerals. 
Streak, brown. Streak, pale yellowish white. 
Taialadant on the edges, to} Semi-transparent to translu- 
opake. cent. 
‘Hardness = 4.75. _.|) Hardness, 5.25 (readily 
ss ay Iscratches Pyrochlore). 
Gravity =4.20 to 4.25. _ Gravity =5.485 to 5.562.* - 
I consider it ‘apparent therefore, that all attempts to > unite two 
the passage of jinsensible sthdatiot? in properties S so “essenti ial as 
specific gravity, internal structure and h | 8s, , must prove una- 
Te 
* The specific gravity as heretofore quoted, wane to 5.00; but voy oppo 
tunely for this examination, Thad bro sught with me from my Charleston etbinet 
es ee crystal, weighing 3.805 grains; and possessing one of Rosinson 
nch-beam. belances Gahiek te turns readily with one thousandth of a grain), I 
ay? as above. The original determination of this 
nied es Gidenk of a balance much less sensitive, and on a crystal 
: ect fh + Ame id four tenths of a grain. Indeed, this was the largest 
erystal | had seen, when I described the speci Observing however, that my 
srystal of 3.805 grains had two of its vest: Sides pitted from the implanta- 
: ce of calcareous spar and green tourmaline, I at first took its eae © gravity with 
these impurities attached. ‘The result was 5.485. I then cleaved off these faces, 
the: sag it Mar ssi ts - ae to 2.410 grains, when ite gravity rose 
ie 
