Delessite is a mineral, a magnesium rich form of Chamosite which is, itself, a member of the Chlorite group Chlorite is a group of phyllosilicate minerals often classified as clays. ...
In the formation of an ordinary agate, it is probable that waters containing silica in solution – derived, perhaps, from the decomposition of some of the silicates in the lava itself – percolated through the rock, and deposited a siliceous coating on the interior of the vapour-vesicles.
The first deposit on the wall of a cavity, forming the "skin" of the agate, is generally a dark greenish mineral substance, like celadonite, delessite or "green earth," which are rich in iron, derived probably from the decomposition of the augite in the mother-rock.
This green silicate may give rise by alteration to a brown oxide of iron (limonite), producing a rusty appearance on the outside of the agate-nodule.
The first layer spread over the wall of the cavity has been called the "priming," and upon this base zeolitic minerals may be deposited.
The first deposit on the wall of a cavity, forming the "skin" of the agate, is generally a dark greenish mineral substance, such as celadonite, delessite or "green earth." These are rich in iron which is probably derived from the decomposition of the agate in the mother-rock.
This green silicate may slowly change to a brown oxide of iron (limonite), producing a rusty appearance on the outside of the agate nodule.
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