
28 STRUCTURAL AND FIELD GEOLOGY 
chalcopyrite (3-5 to 4)—the streak of the former being black, 
while that of the latter is greenish-black. Magnetic Pyrite or 
Pyrrhotite (an iron-sulphide of variable composition) has a 
characteristic pinchbeck-bronze colour, is slightly magnetic, 
and not so hard as pyrite, while the streak is greyish-black. 
Pyrite often occurs in the form of detached crystals and 
aggregates in clay-slate. It is an occasional ingredient of 
schistose rocks, sandstone, coals, and argillaceous rocks of 
various kinds, often as fine-grained impregnations. Now 
and again it appears as an accessory mineral in eruptive 
rocks. It is of frequent occurrence also in lodes, either as 
crystal aggregates or massive. Pyrrhotite is not so common a 
rock-former as pyrite. Occasionally it is present in basic 
igneous rocks (gabbro, basalt, etc.) and schists (amphibole 
rocks). Like pyrite, it often occurs in metalliferous veins—the 
two minerals being not infrequently associated in the so-called 
“bedded veins” or “ quasi-bedded ore formations.” 
Marcasite is an orthorhombic mineral, having the same composition 
as pyrite. It occurs usually compact or cryptocrystalline, and is often 
disseminated in minute grains through certain sedimentary rocks. 
Radiated nodular forms are also very common. The hardness is the 
same as that of pyrite, and the specific gravity slightly less. It is a 
less stable form than pyrite. The colour is bronze-yellow, inclined often 
to green or grey. It has hardly so wide a distribution as pyrite, occurring 
chiefly as concretions in argillaceous and calcareous rocks, 
V, CARBONATES 
Calcite or Calc-spar (calcium carbonate) crystallises in the 
hexagonal system, and assumes a great variety of crystalline 
forms. The cleavage is rhombohedral, as exemplified by the 
well-known transparent Iceland spar, so commonly used for 
polarising instruments; but the unit rhombohedron is a rare 
crystal. Scalenohedral forms are very common, as in dog- 
tooth spar. Calcite is recognised by its slight hardness (= 3), 
as it is easily scratched with the penknife, by the readiness 
with which it effervesces briskly with dilute hydrochloric acid, 
and by its marked rhombohedral cleavage. The specific 
gravity is 2-6 to 2-8. Calcite is an important constituent of 
many aqueous deposits—as limestone, marble (Plate IX. 2), 
calc-sinter, etc. It is a frequent binding material in sedimen- 

