Field book of common rocks and minerals

by Frederic Brewster Loomis

[Ice]
H₂O
[Pl. 51]
water

Occurs solid as ice, snow and frost, or liquid as water; hardness, 2; specific gravity, .92; colorless to white; luster adamantine; transparent on thin edges.

Though we seldom think of ice, and its liquid form, water, as a mineral, still it is one, and perhaps the most important of all minerals, as well as the most common. Ice melts at 32° F. and vaporizes at 212° F., being then termed steam. Because it is so common and liquid at ordinary temperatures it acts as a solvent for a host of other minerals, and is therefore the agent by which they are transported from place to place and redeposited in veins and beds.

Not only does water act as a transportation agent for minerals in solution, but is also the agent of erosion and weathering. Water vaporizes slowly when exposed to the air at all temperatures above freezing, and so it is slowly rising from the surface of the sea or lakes or moist ground into the air, where it would accumulate until the air was saturated, if the air would only keep still and at a uniform temperature. The air will hold a given amount of water vapor, which is, for example, 17 grams per cubic meter when the temperature is 68° F., but at 59° F. it will hold only 12½ grams, or at 50° F. only 9 grams. Thus the air is more or less completely saturated at higher temperatures, and when the temperature is lowered the air can not hold all it has taken up, and it is precipitated in dew, rain or snow, most often as rain. When the rain falls it mechanically carries away, and more or less slowly transports to other places particles of rock, being thus the agent of erosion; and when it is slowed down, as on entering the quiet water of a lake or the sea, it drops the mechanically carried sediment and makes sedimentary deposits.

Another very important and unique feature of water is that on freezing it expands about ¹/₁₁th of its former bulk, so that, as a result, ice floats, and also wherever water in crevices is frozen, the crevices are enlarged. In locations where this freezing and melting take place repeatedly throughout a year, there the breaking up of rocks is rapid.

This is hardly the place to take up a complete discussion of water, but its action as a solvent, mechanically, and in freezing, melting, and vaporizing is the basis of a large part of the study of geology.

When water crystallizes, as in forming ice, it is in the hexagonal system. It tends to twinning and a snow-flake is made up of a large number of twinned crystals, each diverging from the other at 60°. When ice is formed in the air or on the surface of water it forms these complex and beautiful multiple twins, of which but a couple are suggested here. Beneath the surface the hexagonal crystals grow downward into the water, parallel to each other, making a fibrous structure, which is very apparent when ice is “rotten,” which is the time at which the surfaces of the prisms are separating, because the molecules leave the crystal in the reverse order to which they united with it. Frost in marshy or spongy ground will often show this fibrous growth beautifully.

CHAPTER IV
THE ROCKS

Broadly speaking a rock is an essential part of the crust of the earth, and includes loose material, like sand, mud, or volcanic ashes, as well as compact and solid masses, like sandstone and granite. Rocks are aggregates of minerals, either several minerals grouped together, as are mica, quartz and feldspar to make granite, or large quantities of a single mineral, like quartz grains to make sandstone.

The rocks are most conveniently classified according to their mode of origin, into three main groups, igneous, sedimentary, and metamorphic. The igneous rocks are those which have solidified from a molten magma, like lavas, granites, etc. The sedimentary rocks are those which represent accumulations of fragments or grains, derived from various sources, usually the weathering of other rocks, and deposited by such agents as water, wind and organisms. Metamorphic rocks are those which were originally either igneous or sedimentary, but have been altered by the actions of heat, pressure and water, so that the primary character has been changed, often to such an extent as to be obscured.