ACID AND BASIC ROCKS

division of igneous rocks on the basis of their silica content. Chemical analyses of the most abundant components in rocks usually are presented as oxides of the elements; igneous rocks typically consist of approximately 12 major oxides totaling over 99 percent of the rock. Of the oxides, silica (SiO2) is usually the most abundant. Because of this abundance and because most igneous minerals are silicates, silica content was used as a basis of early classifications; it remains widely accepted today. Within this scheme, rocks are described as acid, intermediate, basic, and ultrabasic (in order of decreasing silica content), because it was originally thought by some that silica was present in rock magmas in the form of silicic acid. In modern usage the terms should not be construed to indicate acidity in the old sense, nor do they indicate hydrogen ion concentration (as typically used in chemistry). The terms are well established, however, and can be useful in describing the silica content of rocks. In a widely accepted silica-content classification scheme, rocks with more than 66 percent silica are called acid; those with between 52 and 66 percent silica are intermediate; those with between 45 and 52 percent silica are basic; and those with less than 45 percent are ultrabasic. Compilations of many rock analyses show that rhyolite and granite are acid, with an average silica content of about 72 percent; syenite, diorite, and monzonite are intermediate, with an average silica content of 59 percent; gabbro and basalt are basic, with an average silica content of 48 percent; and peridotite is an ultrabasic rock, with an average of 41 percent silica. Although there are complete gradations between the averages, rocks tend to cluster about the averages. In general, the gradation from acid to basic corresponds to an increase in colour index (dark-mineral percentage), which has led to the development of a parallel, but not equivalent, classification system from felsic to mafic (i.e., light to dark). The fine-grained or glassy nature of many volcanic rocks makes a chemical classification such as acid-basic very useful in distinguishing the different types. Silica content is especially useful because the density and refractive index of natural glasses have been correlated with silica percentage; this makes identification possible in the absence of chemical data. For similar determinations, glasses can also be prepared in the laboratory from crystalline rocks. The influence of silica content on the particular minerals that crystallize from a rock magma is a complex interaction of several parameters, and it cannot be assumed that rocks with the same silica content will have the same mineralogy. Silica saturation is a classification of minerals and rocks as oversaturated, saturated, or undersaturated with respect to silica. Acidic rocks are commonly oversaturated and contain free quartz (SiO2), intermediate rocks contain little or no quartz or feldspathoids (undersaturated minerals), and basic rocks may contain abundant feldspathoids. This broad grouping on the basis of mineralogy related to silica content is used in many modern classification schemes.