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
CategoryPhyllosilicate
Formula
(repeating unit)
NaSi7O13(OH)3·4(H2O)
IMA symbolMgd[1]
Strunz classification9.EA.20
Crystal systemMonoclinic
Unknown space group
Space groupC2/m (no. 12)
Unit cella = 7.22 Å, b = 15.70 Å,
c = 6.91 Å; β = 97.27°; Z=1
Identification
ColorWhite
Crystal habitMinute platy crystals; spherulitic aggregates; powdery
TenacityPuttylike
Mohs scale hardness2
LusterVitreous - dull
StreakWhite
DiaphaneityTranslucent to opaque
Specific gravity2.25 calculated
Optical propertiesBiaxial
Refractive indexnα = 1.470
Ultraviolet fluorescenceYellow-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]

References


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