Carbonate of Iron
The singular specimens here figured are not common, nor have such been much noticed; they are, however, instructive, for which reason I think them proper for this work, and the series I have given is selected rather with a design of shewing the changes taking place under ground, although I bring two different genera under our cognisance in one plate.
The first fibrous radiating specimen I noticed, accompanied the lenticular variety figured in tab. 198; it was of a light brown colour, and the cross fracture exhibited a bronzelike lustre; sometimes) each fibre, at others the whole body of fibres had a thin coat of nearly white Calcedony, and in some parts a few fibres penetrated into Calcedony of a. red colour. I have since recognized a specimen, formerly Mr. Day’s, in which the Carbonate of Iron is blackish, and coated with dull Calcedony; it had often been taken for schorl.
The upper figure is from a very pure variety; it is blacker than the one just mentioned, and is free from Calcedony; the form of the crystal is distinctly a very acute or very much elongated rhomb, with a fracture similar to that of carbonate of Lime, Some of the crystals have their apices truncated: among them is a Small portion of pyrites, as is shewn in the magnified figure. It is probable that this has been produced by the decomposition of pyrites; it seems to occur in a situation where decomposition and change are constantly going on. The lower figure exhibits the calcedony as it remains after the carbonate of Iron, of the kind I first mentioned, has decayed: a single specimen often exhibits, when turned about, many stages of this decomposition, and the various forms of the crystal as retained by the calcedony. I have given a magnified representation of a few of these collected together below. It sometimes happens that the pyrites remains among the calcedony, when it is difficult to decide whether the calcedony derives its form from fibrous pyrites or carbonate of Iron, or whether the pyrites have filled the spaces left by the latter; but a series of specimens explains it. There are some lenticular crystals, which serve as supports to the fasciculi; quartz and copper pyrites occur upon both specimens: they are from the same mine as the large specimen before mentioned (tab. 198): in describing that formerly I was misled by the appearance of the pyrites for want of more specimens.