Ilvaite is a sorosilicate of iron and calcium with formula: CaFe²⁺
₂Fe³⁺
Si
₂O
₇O(OH). Both manganese and magnesium substitute in the structure. Ilvaite crystallizes in the monoclinic system in black prismatic crystals and columnar masses. It is black to brownish black to gray and opaque. It has a Mohs hardness of 5.5 to 6 and a specific gravity of 3.8 to 4.1. Ilvaite is structurally related to lawsonite.

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Ilvaite
General
Category	Sorosilicates
Formula (repeating unit)	Ca(Fe2+)2Fe3+[O|OH|Si2O7]
IMA symbol	Ilv[1]
Strunz classification	9.BE.07
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/a
Identification
Mohs scale hardness	5.5–6.0
Streak	black
Specific gravity	3.8–4.1

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It occurs in contact metamorphic rocks and skarn ore deposits. It also occurs less commonly in syenites.

Ilvaite was first described in 1811 on the island of Elba and the name ilvaite from the Latin name Ilva of the island. Sometimes referred to as yenite.