Feldspar: A Common Mineral Group

Plagioclase Feldspars

• Plagioclase feldspars are the most common feldspar minerals due to their widespread occurrence in igneous, metamorphic, and sedimentary rocks.
• They maintain charge balance during Na+ and Ca2+ ion substitution through the incorporation of additional Al3+ ions in the structure, allowing for negative charge balance.
• The amount of potassium that can enter the alkali feldspar lattice is limited by the lattice dimensions, which restrict larger ions like K+ due to their size compared to Na+ and Ca2+.

Classification of Feldspars

• Plagioclase feldspars are classified based on their composition, primarily with respect to the ratios of Na and Ca, while alkali feldspars are classified primarily based on the presence of K+.
• The general trend in plagioclase composition shifts from Na-rich minerals in granitic rocks to Ca-rich minerals in mafic rocks.
• Boundaries between alkali feldspar minerals on the ternary diagram lack structural significance as they do not correspond to distinct changes in crystal structure.

Etymology and Composition

• The term 'Feldspar' derives from German, meaning 'field stone,' as they were originally found in many field settings.
• The term 'Feldspar' was introduced in the 18th century to categorize the group of minerals characterized by their shared structure and composition.
• The compositional endmember of feldspar that contains calcium is anorthite.

Intermediate Compositions

• There are no feldspars with compositions intermediate between K-feldspar and Ca endmembers due to the dominance of the specific ionic sizes and crystal preference in the structure.
• Celsian is the feldspar that contains barium (Ba) in its crystal structure.
• Celsian and hyalophane are considered volumetrically restricted because their occurrences are relatively rare compared to other feldspar types.

Specific Feldspar Characteristics

• Microcline is best described as a low-temperature polymorph of potassium feldspar with a distinct microcline structure.
• Alkali feldspar found in volcanic and subvolcanic rocks typically includes sanidine due to its formation from rapid crystallization.
• The structural state of alkali feldspars is dependent on temperature conditions during crystallization.
• Adularia, a type of K-feldspar, is characterized by its twinning and occurs in veins and cavities.
• Plagioclase occurs commonly in plutonic rocks, reflecting its formation under slow cooling conditions within the earth's crust.

Learn about the origin and meaning of the term 'feldspar', its introduction as a specific term in the 18th century, and its abundance as a mineral group. Explore the influence of the English word 'spar' and the properties of feldspar minerals.