4 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.60. 



presence of the small amount of plaster of Paris and the bit of sheet 

 iron, but the weight of these is so small relative to that of the stone 

 that the value 3.270 may be accepted as that of the density of the 

 Tuxtla jade, with a possible maximum error of ± 0.001 or 0.002. This 

 density proves at once that we have to do, not with nephrite (the 

 density of which is about 3), but with jadeite, though of a density 

 slightly lower than that of the purely sodic mineral (3.3), and there- 

 fore presumably of abnormal composition. 



Microtexture. — In thin section the jade is seen to be composed en- 

 tirely of grains of clear, perfectly colorless jadeite, not a single par- 

 ticle of any other mineral having been detected in the thin section 

 that was made. No zonal structure is seen in any of the grains. 

 The texture is decidedly granular, but with some tendency to por- 

 phyroblastic texture. Some of the larger phenocrystic grains show 

 an approach to automorphic outlines, and reach lengths of 1-3 mm. 

 But the great majority of the grains are quite anhedral, and much 

 smaller, from 0.1 'to 0.5 mm. in diameter. Between these, here and 

 there, are still smaller grains and granular fragments. All the grains 

 are much cracked.^ The rock has evidently undergone considerable 

 crushing and possibly some slight recrystallization; but the grains 

 are oriented quite at random, and there is no indication of schistosity 

 or a fibrous texture. In short, the rock has the granular texture that 

 is characteristic of jadeite, and not the fibrous texture of nephrite, 

 a distinction that was pointed out many years ago by Clarke and 

 Merrill." 



Optical characters. — Doctor Merwin reports as follows regarding 

 the optical characters of the material: "Several measurements of 

 the refractive index /3 indicated the crystals to be exceptionally homo- 

 geneous and uniform. Because of excessive crushing the optic axial 

 angle was not sharply defined; it was estimated at 70^ to SO'', and 

 was found to be positive. Measurements of « A c varied between 43° 

 and 47°, with the optic plane in the plane of symmetry. The ob- 

 served refractive indices were: a = 1.666, (3— 1.674, 7 = 1.688, 7 — 0:== 

 .022. The optical properties of the mineral, as well as the chem- 

 ical composition, agree with a pyroxene intermediate bet\veen diopside 

 and jadeite." 



Chemical composition. — Material for the chemical analysis, for the 

 thin section, and for the optical determinations, was obtained by 

 sawing two inclined cuts, about 1 inch long and one-third of an 

 inch apart, in the bottom of the figure, giving what seemed to be 

 average material. About 3 grams in all were removed, much of 

 which was lost as dust, and two pieces were used for the section and for 



' The granularity aud the cracked condition of the grains would account for the lack of toughness. 

 » F.W. ( lurke and G.P. Merrill, Proe. U. S. Nat. Mus., vol. 11, pp. 11.5-130, 18S8. 



