The relative abundance of sodium and calcium in melts varies greatly, and the full series of mineral compositions between pure sodium plagioclase and pure calcium plagioclase does occur. If a sodium ion with a 1+ charge substitutes for a calcium ion with a 2+ charge, a balancing substitution of an aluminum ion with a 3- charge for a silicon ion with a 4- charge will also occur. As a result, most albite will contain some calcium substituting for sodium in its crystalline structure, and most anorthite will contain some substitution of sodium for calcium in its crystalline structure. In nature, many melts contain abundant sodium or calcium, along with abundant aluminum, silicon and oxygen. Both of them form by crystallization from a melt. Albite is a sodium feldspar, and anorthite is a calcium feldspar. Albite and anorthite have an interesting relationship. The official field numbers and rock definitions can be found in the description of the figures below, or see Coarse-grained ("plutonic") crystalline igneous rock or Fine-grained ("volcanic") normal crystalline igneous rock. Some of the fields are assigned to 2 or 3 rock types, which are further distinguished by their mafic mineral content or plagioclase type ( calcic or sodic). They mostly have similar field divisions, so most volcanic rocks have a corresponding plutonic rock of equivalent mineralogy (for example, rhyolite and granite). There are 2 QAPF diagrams, one for coarse grained ( plutonic) or intrusive igneous rocks, one for fine grained ( volcanic) or extrusive igneous rocks. The QAPF diagram is divided into 15 basic fields that define ranges of mineral compositions for the different classes of rocks. The corners represent cases in which only one felsic component is present, effectively 100% of either quartz, alkali-feldspar, plagioclase or foid.īecause foids and quartz are mutually exclusive in an igneous rock, the QAPF classification is always based on a maximum of 3 components, either QAP or APF, and the compositions of the rocks are plotted in either the upper or lower triangle. It is made of two ternary diagrams with the corners Q, A, P and F, A, P, adjoined to each other along their A-P edge. The composition of the rocks are plotted into a diamond-shaped coordinate system, the QAPF diagram, also known as Streckeisen diagram (named after the author of the original article). Within the QAPFM classification scheme proposed by the IUGS, rocks with less than 90 vol-% mafic minerals are classified by their content (in volume-%) of:
0 kommentar(er)
