586 PROCEEDINGS OF THE PHILADELPHIA MEETING 



DETERMINATION OF BRUCITE AS A ROCK CONSTITUENT 

 BY ALEXIS A. JULIEN 



i Abstract] 



Reference is first made to the increasing number of rock outcrops at which brucite 

 is a notable accessory or an essential or even predominant constituent, in one 

 recent instance 60 per cent. 



The published data for its recognition by the petrographer are yet limited, in- 

 sufficient, and in some respects inaccurate. 



No reference to twinning is made in the general descriptions, but twins undoubt- 

 edly occur, having been noted in brucitic limestone (predazzite). 



Inclusions, though rare, comprise red and black particles of iron oxides and 

 black needles, and, at one locality, abundant seams or bands of liquid cavities. 



The eminent basal cleavage is the only one recorded, but a distinct rhombohedral 

 cleavage also commonly occurs, and also a third cleavage in traces. 



The parting passes progressively at most localities from a ribbon-like banding 

 to a fasciculate structure. A distinct fibration marks the steps of molecular rear- 

 rangement of foliated into fibrous brucite (nemalite). 



The percussion figure, as produced by a needle, is characteristic for the mineral, 

 partly for the rays diverging at 60 degrees and 30 degrees (as shown by O. Mugge) , 

 but mainly for the indentation or pit, which may be 6, 8, or 12 sided (a negative 

 dihexagonal pyramid), and for a peculiar border of overlapping zones, circular, or 

 sometimes hexagonal. The uniaxial interference-figure, obtained in convergent 

 light, is disturbed in vicinity of the pit, opening into a biaxial figure whose axial 

 plane, in positions around the pit, is found to radiate from that center. 



The low refractive index of brucite facilitates its distinction by the Becke method 

 from a number of its common mineral associates. 



Its strong and positive birefringence, the directions of its two axes of depolar- 

 ization, and its interference- figure, easily obtained, especially in cleavage flakes of 

 the mineral, sometimes when enclosed in rocks, are characteristic toward recognition 

 of brucite. 



The marked strain phenomena of the mineral are described, but the strain bands 

 have been found at only one or two localities, and can not be depended on for de- 

 termination of the mineral, as might be inferred from some statements, though 

 the disturbance of the interference figure is of general occurrence. 



The peculiarities of nemalite proper are described, and of the distinction of two 

 classes of its fibers, and of the effects of flexure, torsion, and strain on its optical 

 properties. The common distribution of nemalite is explained. 



The intimate interlamination of brucite and serpentine are described and the 

 methods for establishing this rather difficult discrimination. 



The chemical methods for its detection are then considered, particularly Lem- 

 berg's test with silver nitrate solution, and a simpler and more accurate modifica- 

 tion proposed. 



The etch-figures by weak acids are found to show characteristic features, though 

 akin to those of dolomite. When mature, the rhombohedral cleavage becomes 

 beautifully etched out. 

 A complete bibliography of the mineral is appended to the paper. 



