CHEMISTRY - Na2[(Fe,Mg)3Fe2]Si8O22(OH)2
CRYSTALLOGRAPHY - Monoclinic
CRYSTAL GROWTH AND HABITS - Typically prismatic crystals but most commonly as fibrous or earthy masses.
COLOR AND OPTICAL PROPERTIES - Black to dark blue. Translucent
HARDNESS - 6
SPECIFIC GRAVITY - 3.2 - 3.4
LUSTER - Vitreous to silky
STREAK - Black
BREAKABILITY - Very good on {110} producing angles of 56o and 124o. Fracture is conchoidal to uneven; Brittle.
OCCURRENCE - Found in alkali granites and syenites.
ASSOCIATED MINERALS - Aegirine, Nepheline, Albite, Arfvedsonite, Limonite, Magnetite, Hematite, Akerite, Siderite, Calcite, Quartz.
MINERAL NAME - In honor of Emil Riebeck, a German explorer.