30 



KEConns OF the Australian MrsETM. 



limouite, or, in one ease, with a dull, steel-grey crust of psilomelane. 

 The mimetite is well crystallised, and two somewhat different habits can 

 be I'ecognised ; the larger crystals are tabular on the base, the prisms 

 being short or absent, and measure up to 20 mm. in diameter, but these 

 crystals ai-e usually coarse and have a frosted appearance. The smaller 

 crystals are j)i'ismatic and attain a length of 6 mm. with a diameter of 

 about 3. Most of the crystals aj'e almost white, with a slight yellowish 

 or buff tinge, but on one hand specimen the mineral is distinctly 

 yellowish and on another green. The crystals on these two specimens 

 are thin tabular on the base and imperfectly formed, many consisting of 

 an aggi'egate of parallel or sub-parallel groupings. 



Nine crystals were measured, and the following forms, one of which, 

 /• (1012), is new, were identified -.—r (0001), >// (lOlO), r (1012), 

 .'' (lOTl), // (2021). Of these the base is never wanting and is usually a 

 fairly large face ; ni is generally well developed though slightly striated 

 hoi'izontally, or it may be quite small or absent altogether; of the 

 pyramids .»■ is always present, y was found on three crystals and r on 

 eight, both as long uari'ow faces. The new form r (10l2) has been 

 recorded for the closely allied species apatite and pyromorphite, but 

 apparenth' has not hitherto been found on mimetite or vanadinite ; the 

 co-ordinate angles for r were found to be cf) 0° 4', p 22° 49' (calculated 

 0° 0' and 22° 47'). These figures are the means derived from 2;> 

 measurements, the limiting values for p being 22° 9' and 23° 16' ; the 

 foi'm may be considered established. 



Angles : 



To coutiini the identitication of the mineral as mimetite a quanti- 

 tative analysis was made of a crystal which appeared to be homogeneous 

 and pure. For estimation of the lead, arsenic and phosphorus, 1-1024 

 gram was dissolved in caustic potash, the lead precipitated by ammonium 

 sulphide and weighed in a Rose ci'ucible. Ki'om the filtrate sulf»hide of 

 arsenic was precipitated by addition of hydi'ochloric acid, tiitei'ed, washed, 

 and oxidised by strong nitric acid. The arsenic acid was precipitated by 

 addition of annnonia and magnesia mixture, and weighed as magnesium 



