1888.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 12,'5 



If the water in No. 1 be regarded as replacing alkalies, the mineral 

 approximates very nearly in composition to normal jadeite, AINa (Si0 3 ) 2 . 

 Both analyses fit in well with Damour's series. 



Under the microscope No. 1 resembles No 59927 from Costa Rica, to 

 be described further on. It is a granular aggregate of colorless or 

 greenish crystals at very imperfect outline, none pleochroic, but polar, 

 izing in very brilliant colors. It has, however, a much coarser texture 

 than the Costa Rica specimen ; the larger granules measuring at times 

 2 millimeters in diameter. These larger forms are all monocliuic with 

 the optic axis in the plane of symmetry, and give extinction angles on 

 sections parallel with the clinopinacoid varying from 35° to 40°. Prom- 

 nent prismatic cleavages are developed, which in basal sections cross 

 at nearly right angles. Many of the crystals also show twin lamellae 

 and carry numerous fluidal inclusions with rapidly moving bubbles. 



The striking feature of the section is that the granules are all badly 

 shattered and traversed by irregular fractures, which, with a power of 

 seventy-five diameters, appear somewhat like the irregular canals of 

 serpentinous matter so often seen in altered olivine. Under a power 

 of one hundred and seventy diameters it becomes apparent that they 

 are undergoing alteration into a fibrous nearly colorless product re- 

 sembling the common change of augite into fibrous hornblende as seen 

 in basic rocks. The alteration begins with a fraying out along the 

 lines of cleavage and fracture, and has in a few instances gone on till 

 but a rounded granule remains of the original mineral. 



In many instances these veins of fibrous material carry plates of a 

 clear, colorless, biaxial, eminently micaceous mineral, showing between 

 crossed nicols the peculiar blistered appearance and brilliant iridescent 

 polarization colors of muscovite. From their small size I am unable 

 to say whether they are in all cases a product of alteration of the pyr- 

 oxene or original iuciosure. I am, however, inclined to the former hy- 

 pothesis, since they occur only along lines where the alteration is great- 

 est and have not been observed in the perfectly unaltered mineral. 

 In some of the larger jadeite objects of the Museum collection from this 

 same locality the micaceous mineral appears in flakes of such size as to 

 be macroscopieally recognizable in the form of minute silvery white 

 inelastic scales. The chemical composition of the rock as a whole is 

 such as to indicate that they are paragonite rather than muscovite. 

 Where these veins are widest the interior is often occupied by a clear 

 and perfectly colorless biaxial mineral without cleavage or crystalline 

 outline, which polarizes in brownish or yellow colors, and which shows 

 the same optical orientation over considerable areas, thus giving rise to 

 what may be called a pseudo-ophitic structure, the grains of still una- 

 tered pyroxene representing the inclosures. Fig. 4 shows the struct- 

 ure of this rock as it appears under a power of twenty-five diameters. 



