Specific gravity: 2.573 calculated
Colour: Pale greenish yellow to colourless
Luminescence: Not fluorescent under UV
Kruijenite is a relatively new mineral, approved in 2018 and to date (June 2022) reported only from the type
At the type locality, Feuerberg, Hohenfels-Essingen, Gerolstein, Vulkaneifel District, Rhineland-Palatinate, Germany, kruijenite was found in fragments of an altered calcic xenolith from volcanic ejecta whose initial size was about 20 cm. The xenolith is composed of fluorite, calcite, aragonite, cuspidine, magnesioferrite, hematite, sharyginite, harmunite, and a hydrous calcium-magnesium-aluminium silicate. It contains cavities encrusted with calcite. The minerals in the xenolith are products of pyro-metamorphic and further hydrothermal alteration of a calcic xenolith under exposure to volatile components of magma.
Kruijenite is one of the latest minerals in this association. The following sequence of mineral formation was proposed:
sharyginite + harmunite → magnesioferrite → fluorite → cuspidine → calcite + kruijenite + hydrous calcium-magnesium-aluminium silicate.
Kruijenite forms long, prismatic, pale greenish yellow to colourless tetragonal crystals up to 0.1 mm × 1 mm. These crystals occur in cavities and are typically combined in radiating or random clusters up to 2 mm in size, as well as dense aggregates intergrown with calcite (AM 106.162).
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