1909-10.] The G-lenboig Fireclay. 357 
Insoluble in dil. HC1 and H 2 S0 4 .... 8*06 
C0 2 33-26 
CaO 1*56 
MgO ...... 339 
FeO 46-45 
Fe 2 0 3 6-49 
H 2 0 (by difference) . . . '79 
10000 
= CaC0 3 ..... 2-78 
MgC0 3 7-08 
FeC0 3 74-97 
84-83 
These determinations established the position of this mineral, in con- 
junction with the microscopic evidence, as a member of the hexagonal 
carbonates intermediate between siderite and dolomite. The mineral 
contains an insoluble residue of 8’06 per cent, and 6*49 per cent, of iron 
peroxide. Its essential constituents are the balance of 74-97 per cent, of 
carbonate of iron, 7*08 per cent, of carbonate of magnesia, and 2*78 per cent, 
of carbonate of lime. Of the minerals of the dolomite-siderite series it 
agrees most closely with the sideroplesite of Breithaupt, of which the 
typical material contained 12 per cent, of carbonate of magnesia. Dana 
quotes a sideroplesite from Salzberg with 10*5 per cent, of carbonate of 
magnesia. The insoluble residue and iron peroxide occur as mechanical 
impurities, and excluding them the proportion of magnesium carbonate in 
the Glenboig mineral is 8*3 per cent., of carbonate of lime 3’3 per cent., 
and of carbonate of iron 88*4 per cent. The material is, therefore, a 
sideroplesite in which part of the magnesia is replaced by lime. 
Sideroplesite was first described by Breithaupt * in 1858. He described 
it as occurring as rhombohedral lens-shaped crystals with rounded faces in 
the Halber Mond Mine at Bohmsdorf, near Schleiz, on the eastern border of 
Saxony ; it was found in veins containing quartz and stibnite. 
Professor Louis of Newcastle has described sideroplesite from London- 
derry, Nova Scotia,]* where it is found in veins interlacing with those of 
ankerite. The material occurs there on the western branch of the Great 
Village river on the southern slope of the Cobequid Mountains, and the 
* A. Breithaupt, “ Beschreibung neuer Mineralien,” Berg- und huttenm.-Zeit., vol. 
xvii., 1858, p. 54. 
t H. Louis, “On the Ankerite Veins of Londonderry, Nova Scotia,” Trans . Nova 
Scotia Inst. Nat. Sci ., vol. v., 1882, pp. 47-53. 
