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Formula: KCa(Si3Al3)O12.5H2O
Tectosilicate (framework silicate), zeolite group
Crystal System: Triclinic
Specific gravity: 2.10 to 2.18 measured, 2.20 calculated
Hardness: 3
Streak: White
Colour: Colourless, white
Common impurities: Na
Environments
Volcanic igneous environments
Metamorphic environments
Hydrothermal environments
Localities
At Ettringen, Vordereifel, Mayen-Koblenz District, Rhineland-Palatinate, Germany, willhendersonite was formed by
contact metasomatism of
limestone xenoliths in
basalt. Associated minerals include
gismondine, chabazite,
ettringite, thaumasite,
phillipsite and thomsonite
(AM 69.186-189, HOM).
At the type locality, the Vispi Quarry, San Venanzo, Terni Province, Umbria, Italy, willhendersonite is a
secondary mineral formed by low-temperature hydrothermal
alteration of potassic basaltic lavas. The most dominant of the
zeolites on the type specimen is
phillipsite-Ca, which is intergrown with willhendersonite.
Thomsonite-(Ca) and apophyllite
formed subsequently. On another specimen, the surface of the matrix is coated with brown
phlogopite, upon which occur crystals of
phillipsite-Ca, thomsonite-(Ca),
melilite and willhendersonite. The matrices of the two specimens
consist of forsterite and
clinopyroxene in a fine-grained groundmass of
melilite, nepheline or
kalsilite with phlogopite
marginal to the forsterite. Also present are traces of
perovskite and apatite
(AM 69.186-189, HOM, Mindat).
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