the formula actually is a blend ranging from all sodium (albite) to all potassium (microcline) but albite is also one endpoint in the plagioclase series so we classify it there. this mineral is often called potassium feldspar or k-feldspar because potassium always exceeds sodium in its formula.
feldspar is a see also of plagioclase. in context|mineralogy|lang=en terms the difference between feldspar and plagioclase is that feldspar is (mineralogy) any of a large group of rock-forming minerals that together make up about 60% of the earth's outer crust the feldspars are all aluminum silicates of the alkali metals sodium potassium calcium and barium feldspars are the principal
feldspars (k al si 3 o 8 – na al si 3 o 8 – ca al 2 si 2 o 8) are a group of rock-forming tectosilicate minerals that make up about 41% of the earth's continental crust by weight.. feldspars crystallize from magma as veins in both intrusive and extrusive igneous rocks and are also present in many types of metamorphic rock. rock formed almost entirely of calcic plagioclase feldspar is known
plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. rather than referring to a particular mineral with a specific chemical composition plagioclase is a continuous solid solution series more properly known as the plagioclase feldspar series (from the ancient greek for "oblique fracture" in reference to its two cleavage angles).
feldspar isn’t really a mineral it’s a class of minerals comprising a range of compositions. likewise the term plagioclase is a subclass comprising a yet smaller range of compositions. there is a lot of variability in feldspars. here is a more