Apophyllite always occurs as distinct crystals, which belong to the tetragonal system.

The density and specific heat of the tetragonal form are greater than those of the yellow.

Associated word: quadrigenarious. four-sided, a. quadrangular, square, quadrilateral, tetragonal, tetrahedral. four-sided figure. square, tetragon, quadrangle, rhombus. foursquare, a. quadrangular. fourth, n. quarter. four years, quadrennium.