ALUMINA. 
321 
LOCALITIES. 
This mineral lias been found in clay near New-Haven in Sussex, Eng.; at Epernay in 
France, and at Halle in Prussia. It is believed to occur in the clay on Staten island, and 
elsewhere in this State. Indeed many of our clays and slates, which contain iron pyrites, 
are covered with incrustations or effloresences of the sulphate of alumina. I am not cer¬ 
tain, however, whether the mineral thus found belongs to the present species, or to the sul¬ 
phate of alumine of Boussingault and Phillips, the alunogene of Beudant, as I have not had 
an opportunity to analyze it. It is probably more common than has been supposed, but may 
have been confounded with alum, for the manufacture of which it may be advantageously 
employed, as it is only necessary to add potash to the solution of this salt. 
IDOCRASE. 
[From the Greek si$«, I see, and xpagig, mixture; because its crystals resemble those of several other 
minerals.] 
Idocrase. Hauy, Cleaveland, Thomson, Phillips , Beudant and Shepard. — Pyramidal Garnet. Jameson. ■ 
midaler Granat. Mohs. — Yesuvian, and Egeran. Werner. 
Fig. 266. 
• Pyra- 
M 
M 
Description. Colour green, brown, and rarely blue. It occurs regu¬ 
larly crystallized ; also massive and disseminated. Primary form a right 
square prism, Fig. 266. Cleavage parallel with M, and also with P, but 
less distinct. Fracture uneven, inclining to small conchoidal. Lustre 
vitreous, or vitreo-resinous. From transparent to translucent on the 
edges. Plardness 6.5. Specific gravity from 3.00 to 3.45. Before the 
blowpipe, it is fusible with ebullition into a yellowish or greenish trans¬ 
lucent globule; with borax, it forms a diaphanous glass tinged green by 
iron. 
From the experiments of Klaproth, Von Kobell and Magnus, it appears that idocrase and 
garnet have the same chemical composition. They differ, however, in their crystalline form, 
and this will probably afford the best means of distinguishing these two minerals. Chryso¬ 
lite, olivine and zircon are infusible. 
Composition. Silica 38.85, alumina 21.93, lime 33.61, oxide of iron 5.40 (Kobell). It 
is probably a compound of the silicate of alumina and lime. 
Geological Situation. This is one of the minerals frequently thrown out by volcanoes. 
In the State of New-York, it is found only in primitive rocks, especially in white limestone. 
LOCALITIES. 
Essex County. Prof. Shepard notices the occurrence of idocrase near Port Henry. He 
describes it as having a dark reddish-brown colour, and being in masses of considerable size, 
one of which was obtained, presenting several crystalline faces. It was said to be associated 
Min. — Part II. 41 
