Siderite

This mineral is evidently a mixture of two species; whether either of them be a variety of one already known is at present uncertain. The dark blue veins consist of a substance resembling' the variety of Quartz distinguished as Semi-opal, of a mamillated form, but of a very different colour, and more transparent. Among this, especially upon its surface, is a pale blue pulverulent substance, which is the colouring- matter of the Quartz-like part, the strength of tone being the effect of the transparency. In many parts the blue substance exhibits a Scopiform fibrous structure; and where this structure is most distinct, the internal fibres of each bundle are found sometimes of a deep green colour, possessing some transparency, and a degree of hardness sufficient to resist the knife: hence it is evident, that the blue substance results from the disintegration of a mineral related to Actinolite or Tremolite. The blue colour is not destroyed by acid. The matrix, which has hitherto been called Granular Gypsum, is Carbonate of Lime in rhomboidal crystals, with convex laces, whose angles approach nearly to 90°: from the slow effect of acids upon them it appears that they contain Magnesia. The interstices between the crystals are filled in the opener parts with white Lithomarga: in the more compact parts there is a good deal of the fibrous substance in minute specks, and in its unchanged state, of a dull green colour: there are also a few specks of Copper Pyrites.

This compound mineral has sometimes been referred to DicroVte, but I have not been able to observe a difference of colour, by looking through it in different directions; and as it does not occur in crystals, there do not appear to be sufficient grounds for their union: there is, however, much analogy; and it is highly probable, that Dicroïte, Siderite, and Prasem, will all one day be found to be similar combinations, but of different species, with Quartz.

Siderite has only been found in the vicinity of Golling in Salzbourg.