Prehnite and Gyrolite

Prehnite

Prehnite only rarely forms isolated crystals or small groups of crystals with a tabular or prismatic shape; much more often, in fact, it is found in botryoidal or stalactitic, granular or compact aggregates. It is generally colourless or whitish, but specimens with green, grey-green or yellow-green colourings are also found, although irregularly distributed. Soluble in hydrochloric acid, it releases water when heated and melts in the blowpipe flame.

It forms in the cavities of basaltic lavas and more rarely in the cavities of granites.

Its most common use is gemmological and ornamental.

Gyrolite

This mineral is commonly found in radiating aggregates of lamellar crystals, in spherulites and concretions. White or colourless, it can vary from transparent to translucent. If heated in a closed container it releases water.

Gyrolite is essentially a secondary mineral and its formation depends on the alteration of calcium silicates, associated with apophyllite, and occurs above all in the cavities of basaltic rocks. The spherulites, which have a diameter of up to five centimetres, can be gathered in clusters of about thirty centimetres.