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A "prismatic" cleavage means the breaks run parallel to the prism faces of the crystal, i.e., parallel to the long c-axis, producing elongated, splintery fragments rather than flakes or blocks.
Most foids have three-dimensional framework structures where the silicate tetrahedra are linked roughly equally in all directions. There aren't strongly preferred planes of weakness, so when you hit them, they tend to fracture irregularly rather than split cleanly.
Cancrinite is structurally different in an important way. Its framework contains large channels running parallel to the c-axis (these channels host CO₃²⁻, SO₄²⁻, Cl⁻, and water molecules). Those channels create distinct planes of weakness running lengthwise through the crystal. As a result, cancrinite splits very cleanly along the prism faces — its perfect {10-10} prismatic cleavage — which is unusual among the feldspathoids and is actually a useful diagnostic feature for telling cancrinite apart from nepheline, which it can otherwise resemble in hand specimen.