Formula: (Fe,Mg)2Al3Si5O18
Origin of name: for J. Sekanina, Czech mineralogist
Crystal system: orthorhombic
Crystal class: 2/m 2/m 2/m
Twinning:
Unit cell:
Colour: blue to violet-blue
Diaphaneity: transparent to translucent
Luster: vitreous
Habit: crystals poorly formed, twinned
Hardness: 7 to 7.5
Specific gravity: 2.77
Cleavage: 2; {100} imperfect, {001} parting
Tenacity:
Streak:
Synonyms/varieties:
Comments: series with cordierite
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