Magadiite

Magadiite is a hydrous sodium silicate mineral (NaSi₇O₁₃(OH)₃·4(H₂O)) which precipitates from alkali brines as an evaporite phase. It forms as soft (Mohs hardness of 2) white powdery monoclinic crystal masses.^[3][4] The mineral is unstable and decomposes during diagenesis leaving a distinctive variety of chert (Magadi-type chert).^[7]

Magadiite
Magadiite. Collected in 1990 from Lake Magadi, Kajiado County, Kenya.
General
Category	Phyllosilicate
Formula	NaSi7O13(OH)3·4(H2O)
IMA symbol	Mgd[1]
Strunz classification	9.EA.20
Crystal system	Monoclinic Unknown space group
Space group	C2/m (no. 12)
Unit cell	a = 7.22 Å, b = 15.70 Å, c = 6.91 Å; β = 97.27°; Z=1
Identification
Color	White
Crystal habit	Minute platy crystals; spherulitic aggregates; powdery
Tenacity	Puttylike
Mohs scale hardness	2
Luster	Vitreous - dull
Streak	White
Diaphaneity	Translucent to opaque
Specific gravity	2.25 calculated
Optical properties	Biaxial
Refractive index	nα = 1.470
Ultraviolet fluorescence	Yellow-white under both long and short wave
References	[2][3][4][5][6]

The mineral was first described by Hans P. Eugster in 1967 for an occurrence in Lake Magadi, Kenya, and is also found at Olduvai Gorge, Tanzania.^[4][7] It is also reported from alkalic intrusive syenites as in Mont Saint-Hilaire, Canada.^[3]