28 MR W. F. P. M'LINTOCK ON THE ZEOLITES AND ASSOCIATED MINERALS 



metamorphosed amygdales, by the maimer in which it encloses dissected scraps of 

 seolecite which are sometimes optically continuous, and by the fact that it not 

 infrequently forms true pseudomorphs after that mineral. 



A colourless pyroxene showing good cleavage and normal optical properties is a 

 common mineral in the slides. It seems to have been formed around the inclusions 

 of chlorite so common in the zeolite, for it is often dusted with turbid chloritic 

 material. It occurs in fairly large crystals and plates and sometimes encloses scraps 

 of seolecite [15]. It is identical in appearance to the pyroxene already described 

 as a rare constituent of the vesicles on Mao nan Damh, but from its mode of 

 occurrence in the present instance there can be no doubt that it is a product of 

 metamorphism. Thus, when the epidote shows the core-and-rim structure already 

 described, the pyroxene is frequently enclosed in the secondary epidote, especially near 

 the junction with seolecite where the crystal often ends in a confused aggregate of the 

 two minerals. Again, when the amygdale originally contained albite and seolecite in 

 intimate association, the pyroxene has picked out and partially replaced the seolecite, 

 whilst it never occurs in the albite. It sometimes occurs in prehnite [15, 16]. 



Garnet has also been formed at the expense of seolecite. It occurs either as idio- 

 morphic dodecahedra, not infrequently showing pronounced zonal structure and 

 optical anomalies, or as irregular patches traversing the zeolite. Inclusions are usually 

 plentiful and consist of epidote, prehnite, and dull turbid fibres arranged in a sub- 

 parallel manner towards the centre of the crystal [58]. Sometimes it forms true 

 perimorphs consisting of a shell of garnet, pale brown in colour and with idiomorphic 

 outlines, which encloses a spongy mass of secondary prehnite [43.] 



Thomsonite, though rare, affords an instructive example of the metamorphism of 

 a zeolite containing both soda and lime. It has not been detected on Maol nan 

 Damh, but, as previously mentioned, it occurs on Coire Bheinn, where the sequence is 

 albite, epidote, prehnite, thomsonite, the various minerals showing the relationships 

 already described. On An Gearna and Beinn Fhada the thomsonite shows conversion 

 to prehnite and albite. In some of the slides the fibrous crystals of thomsonite are 

 partially replaced by crystalline aggregates of finely fibrous secondary prehnite 

 in intimate association with albite which forms irregular, turbid masses full of in- 

 clusions. To distinguish this secondary metamorphic albite from the original albite 

 of the vesicle is a matter of some difficulty ; but its absence of idiomorphism, its 

 constant association with* undoubted secondary prehnite, which it not infrequently 

 encloses, and the occasional presence in it of fragments of thomsonite, all go to prove 

 that it has been developed from the zeolite [50, 81]. Dr Harkbe,* in his account of the 

 metamorphosed amygdales of the Skye basalts, mentions that the lime-soda zeolites 

 give rise to lime-soda felspars. So far, I have failed to detect in the vesicles felspars 

 with a refractive index above that of balsam — a fact which is most probably due to the 

 presence of steam in the vesicles during the metamorphism. Under such conditions 



* A 1Iauk.uk, loo. ciL, p. 51. 



