256 MATERIALS AND THEIR ARRANGEMENT 



shows some variation in hardness, as some specimens can be scratched by steel 

 while others cannot. Hematite is red, dull steel blue, or black. Its most char- 

 acteristic feature is the color of the streak, that is, the color of the fine powder left 

 behind when the specimen is drawn across an unglazed porcelain plate, or powdered 

 very finely in any way. The color of the streak or fine powder is dark brownish red. 

 The pigment called Venetian red is merely very finely ground hematite, contain- 

 ing a small amount of clay. 



Limonite occurs in the same sorts of situations as hematite and as an alteration 

 product of many ore deposits containing iron sulphides. It is deposited in some 

 bogs in sufficient quantity to be valuable as an iron ore. Chemically it is the 

 same as iron rust. Limonite has a very wide range of hardness, from soft earthy 

 material, to compact material which cannot be scratched with steel. In color 

 it ranges from light yellowish brown to very dark brown in some cases almost 

 black. The streak is characteristically yellowish brown, no matter what color 

 the specimen has in mass. The common pigment yellow ocher owes its color to 

 limonite. 



Magnetite. Magnetite occurs in igneous and metamorphic rocks, in contact 

 metamorphic deposits, and to a limited extent in veins. It is very hard, and 

 characteristically black in color, except on weathered surfaces, where it is usually 

 coated with rust. The streak is black. Magnetite is most easily recognized by 

 its magnetic properties. It is strongly attracted by a magnet, and may be mag- 

 netized. Naturally magnetized magnetite is called lodestone, and it is from this 

 substance that our term magnet is derived. Magnetite is of comparatively little 

 importance in America as an iron ore, but in some parts of Europe it is a very 

 important ore mineral. 



Other important rock-forming minerals. A few other common rock-making 

 minerals are mentioned here, though most of them do not occur in igneous rocks, 

 except as secondary minerals introduced subsequent to the hardening of the lava. 

 Several of them occur in metamorphic rocks only. Calcite and dolomite occur 

 abundantly in certain sedimentary rocks, but are secondary in igneous rocks. 



Calcite. Calcium carbonate, CaCOs. H. 3; Sp. gr. 2.72. Calcite is a mineral 

 of very widespread occurrence. It is the chief constituent of limestones and 

 marbles (metamorphosed limestones), and occurs as cavity fillings in many kinds 

 of rocks. It is a very common vein mineral and occurs as an alteration product 

 of lime silicates in many weathered igneous rocks. Calcite is rather soft; that is, 

 while it cannot be scratched with the thumb-nail, it is scratched easily with steel. 

 It has a very good cleavage in three directions which may give rise to rhombohe- 

 drons, that is figures like cubes, which have been compressed along one diagonal. 

 It is recognized most readily by its behavior towards acids, which act upon it 

 rapidly, causing an effervescence of carbon dioxide. Some other minerals effer- 

 vesce similarly, but none of the very common ones show such rapid action as 

 calcite. Transparent pieces of calcite show double refraction; that is, if a piece of 

 transparent calcite is placed over a dot on a piece of paper, two dots may be seen 

 distinctly. Most calcite is white or colorless, but some is colored brown, yellow, 

 green or pink by impurities of various sorts. 



Chlorite. A complex, hydrated silicate containing Fe and Mg. H. 2; Sp. gr. 

 2.65-2.75. Chlorite is a secondary mica; that is, it is a mica formed by the action 

 of weathering on certain silicates containing iron and magnesium. It occurs in 

 metamorphic rocks, and as an alteration product in igneous rocks. It is very 



