Jennite is a calcium silicate hydrate mineral of general chemical formula: Ca9Si6O18(OH)6·8H2O.
Jennite occurs as an alteration mineral in metamorphosed limestone It typically occurs as vein and open space fillings as a late mineral phase. It also occurs in hydrated cement paste.
A first specimen of jennite found in 1966 at the Crestmore quarries (Crestmore, Riverside County, California , US) was analysed and identified as a new mineral by Carpenter in 1966 (Carpenter, 1966). They named it in honor of its discoverer: Clarence Marvin Jenni (1896–1973) director of the Geological Museum at the University of Missouri
In contrast to the first analysis made by Carpenter, jennite does not contain appreciable amount of sodium when the Crestmore specimen was reexamined (Gard, 1977).
The structure of jennite is made of three distinct modules: ribbons of edge-sharing calcium , silicate chains of wollastonite -type running along the b axis, and additional calcium octahedra on inversion centers. The hydroxyl groups are bonded to three calcium cations while no SiOH groups are observed (Bonaccorsi, 2004).
Jennite transforms to metajennite at 70–90 °C (158–194 °F) by losing four water molecules (Gard, 1977).