98 Mr. W. Phillips on Skorodite. [Feb. 



from a mine in the neighbourhood of St. Austell," in Cornwall. 

 By the gentleman who transmitted them, they are imagined to 

 be a variety of the arseniate of iron ; but he laments that their 

 scarcity had prevented his ascertaining their composition, and 

 requests the insertion of a notice respecting them in the Annals 

 of Philosophy. 



The largest of these crystals does not exceed in size the head 

 of an ordinary pin, but many of them are so complete as to leave 

 it a matter of doubt whether they ever were attached to a 

 matrix ; a few, however, are deposited on some small fragments 

 of quartz. In form they very closely resemble that of the skoro- 

 dite, given in the third edition of my Elementary Introduction 

 to Mineralogy ; the planes rfe and d>' , are, however, wanting in 

 the second of the following figures, which represents the form of 

 the crystals lately received from Cornwall ; while almost every 

 one of them exhibits the planes c c, which are not observable in 

 the crystals of the skorodite, or in those of the martial arseniate 

 of copper. 



Externally these crystals are of the dark 

 bottle-green colour, very common to some of 

 the prismatic varieties of the arseniate of cop- 

 per ; but this is not in fact the true colour of the 

 substance itself, which, on holding the crystals, 

 or thin fragments of them between the eye and 

 the light, is found by the assistance of a glass 

 to be of the pale blue, so common to the mar- 

 tial arseniate of copper. The dark-green 

 colour arises from the mechanical intermixture 

 of a multitude of very minute specks, of that 

 colour, visible on the surface, and also by 

 transmitted light. 



M on M 120° 



MonA 119 



Mondi 141 



d\ on dv 103 



d\ ondi" 112 



c on h 154 



The first figure represents a right rhombic prism, the primary form of the martial 

 arseniate of copper, the skorodite, and also of the crystals which form the subject of this 

 notice : the planes d\ , d\ , of the latter, generally present several reflections less than one 

 degree apart, indicating each to be a series of planes. 



The foregoing measurements by the reflective goniometer, as 

 well as the form of these crystals, tend to show that they are only 

 a variety of the martial arseniate of copper, which commonly is 

 prismatic, the planes of the prism being the primary planes 

 M M', sometimes associated with the planes^/' and h, and the 

 prism is commonly terminated by one quadrangular pyramid 

 formed by the planes dd' ; but in these crystals, and also in the 

 skorodite, the planes M M' are reduced to small triangles, 

 owing to the presence of both pyramids. 



The martial aiseniate of copper, and its variety the skorodite, 



2' 

 55 





 45 

 36 

 20 



