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EDWARD S. SIMPSON B.E., F.C.S. : 
Caesium and Rubidium — These two very rare 
metals are closely related to lithium, with which they 
are usually associated in nature. Rubidium is not 
known as an essential constituent of any mineral 
species, and caesium only of two, viz., Pollucite (silicate of 
caesium and aluminium) and Worobiewite (caesium-beryl). 
Traces of both metals are of frequent occurrence in lithia- 
rnicas and other lithia minerals. They are concentrated in 
granitic extrusions. According to Vernadsky* “ Caesium 
and rubidium withdraw from the magmas into "the pegmatite 
veins in the aluminous silicates,” 
Neither metal has as yet been detected in this State- 
They are, however, to be expected in the lithium minerals 
already described and in beryl, which has been recorded from 
several localities, vide infra. 
Beryllium -This somewhat rare metal was first detected 
by Vauquelin in 1797, though it was not till a generation 
later that it was separated in the metallic state, similar in 
appearance, weight and many chemical and physical pro- 
perties to aluminium. Neither the metal nor its compounds 
have any practical applications. 
It is an essential constituent of a number of minerals 
of which the best known is beryl (silicate of beryllium and 
aluminium), a mineral recorded up to the present from 
three localities in this State, viz.. Ravensthorpe, Poonah and 
Greenbusli.es. At Ravensthorpe large pale yellow crystals 
are somewhat plentiful in an albitic pegmatite vein. Several 
greenish crystals have been met with in tin wash at Poonah, 
whilst a single small crystal is recorded from a tin lode at 
Greenbushes. No determinations have been made of the 
proportions of beryllium present in these minerals, but 11 
to 15% of the oxide, or 4 to 5% of the metal, is the average 
content of this mineral. 
A much less common mineral, Gadolinite (silicate of 
beryllium, iron and yttrium) occurs at Cooglegong and in 
the Lake Moore district. The composition of the Cooglegong 
mineral is given below, j It occurs in water-worn pebbles 
and in situ in a pegmatite vein : — 
* W. J. Vernadsky : On Worobiewite and the Chemical Composi- 
tion of Beryl. Trans. Mnsee Geol. Pierre le Grand, 1908, 11, 81, 
t B. F. Davis : Jour. Roy. Soc. N.S.W., XXXVI, 286. 
