494 T. L. WATSON— WEATHERING OF ALLANITE 



According to the analyses and calculations tabulated above, the decom- 

 position of the mineral has been accompanied by a total loss from leaching 

 of 74.63 per cent, or three-fourths of the original material. This includes 

 the unusually large percentage loss of each of the principal constituents 

 in the proportions given in column TV, amounting in every case to more 

 than three-fourths of the original ones. All of the lime has disappeared 

 and with it most of the magnesia, the earths of the cerium group, silica, 

 and much of the alumina. The alkalies are present in such small quan- 

 tity in the fresh mineral as to render conclusions concerning them of 

 doubtful value. 



AMHERST COUNTY 



Locality and mode of occurrence. — The occurrence of allanite on the 

 northwest slope of Friar Mountain, in Amherst Count}^, Virginia, 15 

 miles north of Amherst Courthouse, is one of the most notable in this 

 country, since the mineral is found in unusual quantity. The allanite 

 occurs in pegmatite mostly as detached lumps and masses up to 4 or 5 

 pounds in weight. In addition to feldspar and quartz, the associated 

 minerals are magnetite and sparingly the rare mineral sipylite, a niobate 

 of erbium chiefly, with the metals of the cerium group, etcetera. The 

 feldspar is more or less kaolinized from weathering. 



Fresh allanite. — The fresh allanite from this locality very closely re- 

 sembles that from Roanoke County described on pages 488 to 492. It is 

 black in color, has vitreous luster, and shows traces of cleavage ; fracture, 

 uneven; streak, gray; hardness, nearly 6; and specific gravity, 3.83. All 

 of the mineral examined microscopically from the Amherst County locality 

 was birefracting, the optical properties of which were determined and are 

 given on page 481. 



Weathered allanite. — Lumps and masses of the mineral are usually 

 coated with a crust, which has resulted from weathering, varying from a 

 mere film to, in extreme cases, more than a quarter of an inch in thick- 

 ness. Much of the crust is dark reddish brown to brick red in color, 

 sometimes light yellowish to buff, rarely nearly white. In some cases the 

 crust exhibits a rudely concentric structure and is separable into two 

 parts, an outer exceedingly thin layer of nearly white color and an inner 

 one of brick red color, both being pulverulent and more or less earthy in 

 character. This layered structure is not original in the fresh mineral, 

 but is plainly the result of alteration from weathering. 



Separate analyses of the two parts of the crust are given in the table 

 below. The brick red crust dissolves in hot HGl, with the separation of 

 gelatinous silica ; is infusible before the blow-pipe, the residue being non- 

 magnetic; yields water when heated in closed tube, which reacts faintly 



