﻿274 J. G. Meem — Limonite Pseudomorphs after Pyrite. 



Aet. XXXI. — Limonite Pseudomorphs after Pyrite ; by JOHN 



G. Meem. 



Seeing an article in the May issue, vol xxxi of this Journal, 

 on limonite pseudomorphs, it was thought that a short account 

 of the pseudomorphs occurring in Kockbridge County, Va., 

 would be of interest. These crystals are found on a hill 

 sloping gently to a small stream (Hoffman's Eun) about three- 

 quarters of a mile southeast of Lexington, where they occur in 

 the soil, on or near the surface, with outcropping limestones of the 

 Lower Silurian age. In color they vary from a very light to a 

 very dark brown, and in some cases are almost black. Some 

 faces have a fine luster, others none at all. The pseudomorphs 

 are hydrous and give a yellow powder, showing them to be limon- 

 ite. Most of the crystals have undergone a complete alteration, 

 there being no pyrite visible to the naked eye. In some, the 

 unaltered pyrite is found forming a nucleus at or around the 

 center; in others, it forms the bulk of the crystal, the latter 

 then having only a coating of limonite. Bedded in the lime- 

 stones from the same locality, unaltered pyrite occurs, though 

 not in well defined or isolated crystals. 



The forms of these pseudomorphs present some features 

 worthy of notice. The most common is the octahedron. This 

 generally occurs combined with the cube, and of these combi- 

 nations we have every variety from large octahedral and very 

 small cubic faces to the other extreme (see figs. 2, 3, 4). 



One of the points of interest is noticed in the following. On 

 nearly all the octahedral faces (1, Dana) striations running at 

 right angles to the edges of these faces are observed (figs. 4, 5). 

 Col. M. B. Hardin, Professor of Chemistry, etc., called attention 

 to it and suggested that it pointed to a tendency in the crystal 

 to form trapezohedral faces, that is, to an oscillation between 

 the octahedron and trapezohedron. Observations were begun 

 with the view to settle this if possible. Specimens were 

 obtained in which the angles varied more and more from the 

 octahedral angles, and in which the tendency to trapezohedral 

 faces (m-m) was more marked, until finally some were found 

 that had the faces (m-m) fully developed, the striae obliterated 

 and the edges very clean cut (figs. 6, 7, 8, 20, 21). In some 

 of the faces (1) in which there are no striations observable, the 

 angles are distorted from the true octahedral angles by a slight 

 bulging at the center of the faces. Aggregations of these 

 crystals are very common (fig. 1, example of cubes). In some 

 of the crystals whose form is shown to be trapezohedral by 

 projecting edges, this aggregation of the small trapezohedrons 

 is carried to such an extent and in such a manner on the 



