Читать книгу The Circle of Knowledge: A Classified, Simplified, Visualized Book of Answers - Various - Страница 159

There are three general classes of crystals—calcareous, silicious and gypsum—but by far the most important are the silicious crystals because of their great hardness. These include quartz or rock crystal—which is quite common—and the so-called precious stones, among which are the diamond, rubies, sapphires, etc., a description of which will be found in the Dictionary of Minerals.

To find the relative hardness of substances, a scale has been arranged, beginning with the softest mineral (talc) and ending with the hardest (diamond). The minerals of the scale, therefore, are so arranged that each will scratch any other mineral of lower number in the scale, or be scratched by any of higher number.

Scale of Hardness

	Mineral		Chemical Name
1.	Talc.		-	Can be scratched by the finger-nail	-		1.	Magnesium silicate.
2.	Gypsum (or rocksalt).	2.	Calcium sulphate or Sodium chloride.
3.	Calc-spar.		-	Can be scratched by knife or file	-		3.	Calcium carbonate.
4.	Fluor-spar.	4.	Calcium fluoride.
5.	Apatite.	5.	Calcium phosphate.
6.	Felspar.	6.	Potassium and aluminum silicates.
7.	Quartz (rock-crystal).		-	Cannot be scratched by knife or file	-		7.	Silica.
8.	Topaz.	8.	Aluminum fluosilicate.
9.	Corundum (sapphire, ruby).	9.	These gems are crystallized alumina.
10.	Diamond.	10.	Crystallized carbon.

As a first inquiry into the chemical composition of a mineral, dilute hydrochloric or sulphuric acid is tried. All carbonates effervesce when placed in acid or when acid is dropped upon them, while quartz and all the silicates show no effervescense when so treated.

The table on pages 104–7 contains a brief description of the distinctive physical features of a number of the very common or important minerals.