256 



Quantitative Separation into Magnetic Groups. 



The method of examination followed was essentially that recommended by 

 T. Crook.^^^ This comprised a preliminary separation of the constituents into 

 groups of varying magnetic permeability by means of an electro-magnet fitted 

 with adjustable pole pieces for altering the intensity of the magnetic field. 



It was found that the component minerals of the sand could by this means 

 be separated into five well-defined groups, and since in each case a weight of 

 20 grams of sand was used, the relative weights of each group afforded a means 

 of comparing the three types of sand. The following groups were thus 

 recognized : — 



(1) Highly magnetic, represented by . . Magnetite 



(2) Moderately magnetic, represented by . . Ilmenite 



(3) Appreciably magnetic, represented by . . Garnet 



(4) Weakly magnetic, represented by . . Staurolite 



(5) Non-magnetic, represented by . . . . Zircon 



The relative proportions of the various groups in the three types of sand 

 were found to be as follows : — 





Percentage Weights. 







Magnetite Group 

 Ilmenite Group 

 Garnet Group 

 Staurolite Group 

 Zircon Group 



Petrel Cove. 



1 



. . 1-8 



. . 548 



14 



.. 419 



Bluff Bay. 



■5 



3-3 

 57-3 



10 

 379 



Middleton 

 10 



80 

 750 



5-5 

 10-5 



Component Minerals of the various Magnetic Groups. 



The separation and identification of the component minerals was effected by 

 a series of heavy liquid flotations and electrostatic separations, coupled with 

 microscopical examination. 



Owing to the relatively high specific gravity of most of the minerals the 

 use of heavy liquids for separation was necessarily somewhat restricted, but 

 Methylene Iodide (S.G. 33 ) and fused Mercurous Nitrate (S.G. 43) were found 

 very useful in certain cases. 



In some instances it was found that certain of the minerals showed varying 

 magnetic permeability, probably owing to inclusions of foreign material ; this 

 caused them to appear in two or more of the magnetic groups. Separation in 

 these instances was effected by electrostatic methods, a process well adapted for 

 the separation of Ilmenite from the Garnet or Staurolite Groups.*^-) The follow- 

 ing minerals were identified: — 



From Magnetite Group . . Magnetite 



Ilmenite Group . . Ilmenite 



Garnet Group . . . . Almandine 



Staurolite Group . . Staurolite, Green Spinel, Tourmaline 



Zircon Group . . . . Zircon, Kyanite, and Rutile 



Each of the three types of sand was found to contain the same set of 

 minerals, and as the properties of any one mineral were, in general, common to 

 that type at each locality, it is convenient to group the petrological features col- 

 lectively. 



<i) Vide Text-book of Petrology, "The Sedimentary Rocks," by Hatch and Rastall. 

 Appendix by T. Crook. Page 339, ei seq. London, 1913. 

 ■C^) Vide Mineralogical Magazine, vol. 15, p. 26(L 



