Cancrinite

cancrinite [′kaŋ·krə‚nīt]

(mineralogy)

Na₃CaAl₃Si₃O₁₂CO₃(OH)₂A feldspathoid tectosilicate occurring in hexagonal crystals in nepheline syenites, usually in compact or disseminated masses.

---

Cancrinite 

(named after the Russian statesman E. F. Cancrin [Kankrin]), the group name of minerals representing a perfect isomorphic series of mixed crystals with the composition nNa₆Ca₂[AlSi0₄]₆(C0₃)₂⋅2—3H₂O (carbonate cancrinite) + mNa₆Ca₂[AlSi0₄]₆(S0⁴)₂⋅2—3H₂O (sulfate cancrinite, or vishnevite). The Na:Ca ratio varies; often an impurity of K₂O (up to 5 percent) is detected. Cancrinite belongs to the aluminosilicates, with a framework of hexagonal SiO₄ rings and A1O₄ tetrahedrons connected in vertical six-membered groups. In the ring cavities lie large ions of [SO₄]²⁻ or [CO₃]²⁻, formingmolecular intrusive groups with the Ca or 2Na. The mineralcrystallizes in the hexagonal system (crystals are very rare);granular or fine-grained aggregates are common. Cancrinite iscolorless, gray, or pink; sometimes it is light blue (sulfate cancrinite). The prismatic cleavage is perfect. The mineral has a hardness of 5–5.5 on the mineralogical scale and a density of 2, 420–2, 500 kg/m³. In nature it forms from nepheline under the action of sulfate or carbonate postmagmatic solutions. It is found in nepheline syenites and other basic rocks as well as in alkaline pegmatites as a rock-forming mineral.