Paragonite
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Paragonite is a mineral, related to muscovite. Its empirical formula is NaAl₂(OH)₂. A wide solvus separates muscovite from paragonite, such that there is little solid solution along the vector Na⁺K⁺ and ... Wikipedia
Birefringence: δ = 0.036
Cleavage: Perfect on the
Crystal class: Prismatic (2/m); (same H-M symbol)
Crystal habit: massive, fibrous or scaly
Crystal system: Monoclinic
Formula (repeating unit): NaAl2(AlSi3O10)(OH)2
Fracture: Micaeous