The main components of basalt are silicon dioxide, aluminum oxide, iron oxide, calcium oxide, and magnesium oxide (with small amounts of potassium oxide and sodium oxide). Silicon dioxide is the most abundant component, accounting for approximately 45% to 50%. Based on the degree of SiO2 saturation and alkalinity, basalt is divided into two main categories:
1. Tholeiitic basalt (i.e., subalkaline basalt): This is a rock that is supersaturated or saturated with silicon dioxide. It does not contain olivine and nepheline, and is characterized by the presence of orthopyroxene and clinopyroxene. The relationship between its silicon dioxide and total alkali content is (Na2O+K2O)/(SiO2-39) < 0.37.
2. Alkaline basalt: Basalt often exhibits amygdaloidal texture, meaning that circular or oval-shaped vesicles are distributed within the rock. These vesicles are filled with later-formed minerals, forming an amygdaloidal structure. Amygdaloidal texture is a common characteristic of basalt, reflecting the process of gas escaping during magma cooling and being filled with minerals. It is undersaturated in silicon dioxide and rich in alkalis. It contains olivine and feldspathoids (such as nepheline), zeolites, etc. The latter two minerals sometimes occur as interstitial fillings in the matrix, along with alkaline feldspar or potassic plagioclase and potassic orthoclase; it does not contain orthopyroxene or clinopyroxene, but only calcium-rich clinopyroxene, i.e., diopside-rich augite. The value of (Na2O+K2O)/(SiO2-39) is greater than 0.37.
