White acicular crystals of jennite associated to also white ettringite, from Bellerberg, Ettringen, Eifel Mountains, Rhineland-Palatinate, Germany. — Photo: David Hospital · CC BY-SA 4.0
Jennite
A calcium silicate hydrate filling veins in metamorphosed limestone and skarn — a natural analogue of cement hydrate.
Intro adapted from Wikipedia (CC BY-SA).
Jennite is an IMA-recognized mineral species in the silicate class, with the chemical formula Ca₉(Si₃O₉)₂(OH)₆·8H₂O.
It crystallizes in the triclinic crystal system (space group P-1).
Key properties: Mohs hardness 3.5; specific gravity 2.32–2.33; Vitreous luster; refractive index 1.561; optically biaxial.
IMA symbol Jnn, IMA status: approved, named in 1965.
A late-stage mineral, partially filling open spaces and in veins in fractured contact metamorphic rocks.
It is found associated with Tobermorite, scawtite, calcite (Crestmore, California, USA); afwillite, oyelite, spurrite (Fuka, Japan).
The name refers to Colonel Clarence M. Jenni (1896-1974?), Director of the Geological Museum, University of Missouri, Rollo, Missouri, USA, the mineral's discoverer.