BY VV. F. PETTERD, C.M.Z.S. 71 



125. Fayalite (Iron Olivine). 



This variety of tlie common olivine is abundant as micro- 

 scopic crystals, which are of a bright-red colour by trans- 

 mitted light, in the fayalite-melilite basalt which occurs 

 as an extensive sheet at the Alexandria Battery, near 

 Hobart. 



126. Felspar Group (Polysilicates of Aluminium^ Potas- 



sium, (&C.). 



The principal scientific interest attached to this group 

 is the fact that they are the most important of all rock- 

 forming minerals. The classification of the greater number 

 of igneous rocks depends upon the species of felspar form- 

 ing the essential constituents of the mass. The rock-form- 

 ing members of the group are found for the most part only 

 in microscopic crystals or portions of the same, but ortho- 

 clase often occurs of considerable size individually and in 

 large masses. In pegmatite dykes they assume compara- 

 tively gigantic proportions. They are all brittle, have a 

 conchoidal fracture, with ready cleavage in certain direc- 

 tions ; hardness about 6. They decompose to kaolin. They 

 may roughly be divided into two large crystallographic 

 divisions, viz., the monoclinic or orthoclastic and the tri- 

 clinic or plagioclastic. In composition they are silicates of 

 aluminium with either potassium, sodium, or calcium, and 

 rarely barium. Magnesium and iron are never present. 

 They are always of pale shades of colour — from white and 

 translucent to yellowish, red, or green. The specific gravity 

 ranges from 2'5 to 2"9. The felspars are classified first as 

 regards form and next as regards composition. The mono- 

 clinic comprise orthoclase, potash felspar; soda orthoclase, 

 potassium-sodium felspar, and hyalophane-barium felspar. 

 The triclinic species include microcline and anorthoclase, 

 potassium-soda felspars ; albite, sodium felspar ; anorthite, 

 calcium felspar: celsian, barium felspar; as well as the 

 intermediate sodium-calcium or calcium-sodium felspars, 

 oligoclase, andesine 



The following are the more important species of the 

 group as occurring in this island, viz. : — 



Orthoclase occurs in our granites, syenites, elvans, and 

 quartz porphyries. The most common combinations are 

 (010), (liO), (001). Carlsbad twins [twinning plane 

 parallel to the orthopinacoid] (100) are frequently seen. 

 The crystals are generally turbid from decomposition into 

 kaolin, or muscovite. Replacement by pinit-e, chlorite, &c., 

 has occasionally taken place. Porphyritic crystals of an 



