Diagonaalide

Diagonaalide is a rare silicate mineral characterized by a distinctive diagonal cleavage that causes crystals to break along two diagonally oriented planes. It crystallizes in the monoclinic system and forms prismatic to tabular crystals that are colorless to pale green, with a vitreous luster and transparent to translucent appearance. The Mohs hardness is about 6–6.5. The mineral's chemical composition is dominated by calcium and magnesium with silicon and oxygen forming the silicate framework; aluminum and iron commonly substitute for calcium or magnesium in trace amounts. A nominal formula is represented as (Ca,Mg)Si2O6, with trace substitutions.

The SiO4 tetrahedra connect to form a framework, into which layers of alkaline-earth cations are incorporated. The diagonal cleavage is attributed to the arrangement of bonds along specific crystallographic planes, yielding characteristic diagonal fracture surfaces when crystals are broken.

Diagonaalide was first described in the 1990s from hydrothermal veins in metamorphic rocks. The type locality is the Diagonaal Ridge region, and the mineral has since been reported in a handful of hydrothermal and contact-metamorphic settings in Europe and North America. It is typically found with quartz, calcite, diopside, and scapolite.

Because of its distinctive cleavage and crystal habit, diagonaalide is of interest to mineralogical research and collectors. It does not have widespread industrial applications due to its rarity.