( 59 ) 



. Serpentine, H4Mg3Si20,,. — Not infrequent as a deeomposition 

 product of forsterite. 



ApuphylUte, (HK)oCa(SiO;02H,0. 



Topaz, (AlF)2SiO^.— Commonly found in gem gravels ; yellow, 

 colourless, or pink, the latter variety known as king topaz. The 

 colourless varieties are wrongly known as water sapphires. The pale 

 greenish-blue varieties are cut as aquamarine. Not met with in situ. 



Andalusite, Al(A10)Si04. 



Kyanite, (^10)38103.— Has only been found in dredgings made 

 by Professor Herdman oflE the coast of Ceylon. 



SilUmanite, AlgSiO.,. — Very rarely in good crystals in gem 

 gravels. Sillimanite is in some districts a common constituent of 

 the garnetiferous leptynites, the rocks then resembling the 

 khondalites of Southern India, of which specimens are exhibited. 

 Fine coarse sillimanite rocks occur near Haldummulla, the 

 sillimanite being disposed in sheaf -like and radiating aggregates. 

 The associated minerals are corundum, garnet, orthoclase-micro- 

 perthite, ilmenite, and rutile. See also under Corundum. 



KaoUnite {China Clay), H^AUSioOi,.— Common as a decom- 

 position product of orthoclase. 



Felspars : 



(1) Orthoclase, KAlSigOg affords the well-known moonstone ; it 

 occurs in large but well-cleaved crystals in certain acid granulites 

 associated with crystalline limestones in the Dumbara district, 

 Central Province. Various pegmatites also, consisting of quartz 

 and orthoclase, yield moonstone of a poor quality. Large indivi- 

 duals of idiomorphic orthoclase occur in pegmatite veins at Galle. 

 An intergrowth of orthoclase with albite is the most usual felspar 

 of the less basic granulites. The silvery sheen so characteristic of 

 moonstone is probably the result of the presence of excessively 

 minute inclusions of kaolin, the products of incipient decom- 

 position. The bluish- white opalescence of moonstone is best seen 

 when the crystal is viewed in a direction at right angles to the basal 

 plane, i.e., when regarding one of the planes of easy cleavage ; the 

 stone should always be so cut that the flat base of the finished 

 cabochon gem is parallel to this surface, in order that the opales- 

 cence may be central and as conspicuous as possible. 



(2) Plagiodase {Lime-Soda Felspars). — These are commonly 

 characteristic of many of the granulites, but are rarely of large size. 

 A fine blue opalescence has occasionally been observed in the 

 plagiodase felspars, but none of size suitable for yielding gems have 

 been found. 



(3) Jlf icroc^m e, KAlSaOg.— Rarer than orthoclase in the granu- 

 lites. 



