MINERALOGICAL AND CHEMICAL CHARACTERS. I 7 



The crystal-outlines are not always visible in mica, more generally 

 they are not exhibited in ordinary specimens. In such cases the direc- 

 tion of the so-called plane of symmetry can be determined by produc- 

 ing a percussion-figure. For this purpose a thin mica plate is placed 

 on a sheet of cardboard, or similar firm plane surface, and is then 

 struck a sharp blow with a blunt needle point. A six-rayed star is 

 produced in this way, the cracks intersecting at the centre of percus- 

 sion at angles of 6o° approximately. 1 



The ray which lies nearly parallel to the clinopinacoidal plane (that 

 is, to the plane of symmetry) 2 is spoken of as the characteristic, or 

 leading, ray, and this, in the first group of micas (muscovite and its 

 fellows), lies at right angles to plane of the optic axes, whilst in the 

 second group of micas (biotite, phlogopite, etc.) it lies nearly parallel 



are mere restatements of facts known concerning the mica group, the statements 

 are not made without special verification on micas of Indian origin, and some 

 of them represent facts hitherto unrecorded. 



3 The plane of symmetry is referred to as if the micas were all monoclinic in 

 their system of crystallization. Reasons are given below for considering muscovite 

 to be truly monoclinic; but in some other micas the asymmetry of the etch-figures 

 and the slight divergence of the optic-axial plane from what appears to be the 

 symmetral plane of the geometrical crystal indicate triclinic forms. It is possible 

 that the apparently higher type of symmetry is produced by a twinning of a tricli- 

 nic form on a scale too minute for recognition by tests less precise than that 

 afforded by etch-figures. This introduces theoretical questions beyond the scope 

 of this paper ; it is sufficient to recognise the fact that, as in the case of the fel- 

 spars, there is a sufficient homology of crystal-habit recognisable throughout both 

 monoclinic and triclinic types to permit identification of corresponding forms. The 

 expression " plane of symmetry " may thus be not quite correct in the case of some 

 micas, whilst " parallel to " and " perpendicular to " this plane mean approximately 

 parallel, or perpendicular, to the plane which occupies the position of the synv 

 metral plane in a monoclinic mineral. 



3 Tschermak divides biotite into two varieties, meroxene, which includes all 

 ordinary forms met with in crystalline rocks, and anomite (from a-vo/MOi, contrary 

 to law) a rarer variety having its optic axial plane at right angles to the usual 

 direction. 



> It will be seen below that the common assumption ol the mineralogical text- 

 book is not quite correct in referring the angles of the percussion-figure to 6o°. 

 2 See foot-note 2, p. 16. 



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