ALUMINA. 



321 



LOCALITIES. 



This mineral has 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 ISlaten 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 ahmiinc of Boussingault and Phillips, the alunogenc of Beudant, as I have not had 

 an opportunity to analyze it. It is probably more common than has been supposed, Init 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. 



M 



IDOCRASE. 



[From the Greek siou, / see, and xpasi.s, mixture ; because its crystals resemble those of several other 



minerals.] 



IJocrasc. Hauij, Clcai-dand, Thomson, Phillips, Beudant and Shcpanl. — Pyramidal Garnet. Jameson. — Pyra- 

 midalcr Granat. Mohs. — Vcsuvian, and Egcran. U'cnier. 



fig- -CO. 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. Hardness 6.5. Specific gravity from 3.00 to 3.15. 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 liest 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 (KohcU). It 

 is probably a compound of the silicate of alumina and lime. 



Geological Situation. This is one of the minerals frecjuently 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. — P.uiT H. 41 



