6.4 RECORDS OF THE AUSTRALIAN MUSEUM. 



surfaces have a dull yellowish tinge deepening occasionally towards 

 the edges. Internally they are colourless and translucent and 

 shew the rhomboidal cleavage perfectly. 



In the one specimen the crystals of ankerite are associated with 

 large and well formed crystals of quartz, some of which are left- 

 handed, having both the rhombo- and trapezohedral surfaces. A 

 few saddle-shaped crystals of siderite are deposited here and there 

 on both minerals.* 



The other specimen contains no siderite, the associated minerals 

 being quartz crystals, and a few large and numerous small crystals 

 of albite. 



Normal ankerite has the formula 2 CaCOs.FeCOs.MgCOs 

 assigned to it and requires 



2 CaCOj 50-0 



FeCOj ... 290 



MgCO, 21-0 



100-0 



The analysis of the above mineral is seen to differ from this in 

 the relative proportions of the carbonates of iron and magnesia. 

 Calculated however for the formula 5 CaC03.2 FeCOs.S MgCOg 

 the percentages found agree exceedingly well with the theoretical 



Borickyt writes the formula for ankerite and similar minerals 

 thus : — 



(CaFeC^Oc)-^^ (CaMgC,Oe), 



and states that n may vary from ^ to 10. When n is 1 the 

 formula represents normal ankerite, but he assumes that those 

 minerals in which n is 2 or less may be classed as ankerites, 

 while he names tho.se in which n is greater than 2 parankerites. 

 The formula 5 CaCOa. 2FeC03. SMgCOg calculated for the present 

 mineral may be written : — 



2 (CaFeC20e)-t-3 (CaMgC^Oe) 



* These crystals of siderite have already been figured and described 

 by the late Mr. F. Ratte in " Notes from the Australian Museum," Proc. 

 Linn. Soc. N. S. Wales, x., 4, 1885, p. 759. 



t Boricky— Min. Mitth., xlvii., 1876. 



