30 N. L. Bowen — Genetic Features of 



pure lime olivine itself (/?Ca 2 Si0 4 ) is known to. occur in 

 such rocks, though it has frequently been stated that 

 this compound is unknown as a rock mineral. ^ F. P. Paul 

 describes an unknown mineral in a nephelite-eudialite 

 basalt from Shannon Tier, Tasmania. 24 His chemical 

 determinations are sufficient to prove that the mineral 

 is a lime-silica compound high in lime and free from 

 magnesia, and he himself decides that it is probably 

 Ca 2 Si0 4 . That he is certainly correct is demonstrated by 

 the correspondence of properties with those of artificial 

 /?Ca 2 Si0 4 , 25 which is shown below. 



Cleavage 7 a 2V Sign 



£Ca 2 Si0 4 poor 1.735 1.717 large + 



Tasmanian mineral 2. oli vine-like 1.746 1.718 64° + 



Petrographers. appear to have entirely overlooked this 

 occurrence of lime olivine. 26 Its nearest relative among 

 recognized minerals is hillebrandit-e, Ca 2 Si0 4 .H 2 0. 27 The 

 occurrence of Ca 2 Si0 4 as a mineral in an igneous rock 

 may be regarded as further justification for the use of 

 this molecule in calculating the norm. 28 



The compound Ca 2 Si0 4 occurs in three forms. The 

 /?-form with which the natural mineral corresponds is 

 stable only between 1420° and 675°, and when pure it 

 changes at room temperature (or higher) to the y-form, 

 the change being accompanied by such increase of volume 

 that the whole mass is reduced to powder. When embed- 

 ded in a mass of glass or other minerals this change is 

 prevented and in such a case it will persist and its proper- 

 ties can be studied at room temperature. Its occurrence 

 as grains in the natural rock is, therefore, not contrary to 

 laboratory experience. It may be noted that the artificial 

 mineral is strongly attacked by water with solution of 

 lime and it would be of interest to test the Tasmanian 

 rock in this particular. According to Paul the natural 

 mineral is readily attacked by very dilute HC1. 



The rock in which this mineral occurs is a local varia- 

 tion of melilite-nephelite basalt. A reference to fig. 1 

 shows that the field of Ca 2 Si0 4 borders against the aker- 



24 T. M. P. M., 25, 309, 1906. 



25 Day, Shepherd and Wright, this Journal, 22, 295, 1906; also Rankin and 

 Wright, ibid., 39, 75, 1915. 



28 For example, in Rosenbusch, Mikroskopische Physiographie (1908), p. 

 1449, it is still described as an unknown mineral. 

 27 F. E. Wright, this Journal, (4) 26, 551, 1908. 

 2S H. S. Washington, J. Wash. Acad. Sci., 5, 345, 1915. 



