The geodes of Okenite.

A small white ball of flexible needle-like crystals , associated with zeolites , which appears as a contact mineral in basalts ... a hydrated calcium silicate : the okénite .

            In the center, Okénite on gyrolite - Origin: Mumbai / India - Photo Kluka

These white crystals of formula  Ca₁₀Si₁₈O₄₆.18H₂O are extremely fragile and the slightest touch leaves traces . They are found mainly in Pune (or Poona ) in India in the Deccan Traps ( 60-68 Ma) , also in Greenland ( Disko island ), Chile and Ireland.

              Deccan Trapps  - view of the stack of lava flows near Pune / India - Photo Kppethe .

The first description was made in a discovery in 1828 on the Greenlandic Disko Island and its name, " ockenite " later turned into okénite , honors a German biologist Lorenz Ocken .
The most attractive specimens are accompanied by rare minerals such as gyrolite or apophyllite , the phillosilicates .

Some volcanic vesicles ( bubbles in the basalt) are lined with delicate tufts of okenite , and are called geodes of okénite .

Ultra sensitive , never to be touched , they are difficult to conservation .

                         Okenite geode  in basalt - Photo Amazing Geologist

Geode in basalt containing Okenite - Origin: Khandivali , Maharashtra , India ( 28 × 22 cm) - Photo Didier Descouens
 Okenite - enlargement of the central part of the geode / photo above - Origin: Khandivali , Maharashtra, India ( 28 × 22 cm) - Photo Didier Descouens

Geodes , from the Greek γεώδης - ge- odes, like land , can form in any type of rock or mineral , but out of habit , the term is usually reserved for training in sedimentary, vein, magmatic and volcanic rocks.

The formation of geodes in volcanic rocks :

Between 800 ° C and 1200 ° C , molten lava dissolved volatile components of the rock and produces gas bubbles. As the gas bubbles are moving in the heart of the lava , they migrate to the surface and can come together to form pockets of larger and larger . Cooling lava, synchronous of the process, latter freezes the gas bubbles from a certain threshold of viscosity. They shall keep a lens shape.

                                           Geode - ulaval.ca scheme

During cooling of the liquid , the gas bubbles are transported in the direction of flow of lava . Time factors and management of the flow during his cooling characterize the shape lens that takes the bubbles. These lenses are like drops, thin spikes facing upstream of the flow. The observation of geodes in a lava flow allows to assess with some precision the direction of flow , the origin and rate of cooling temperature and the nature of any obstacles destroyed by the furnace of lava that can be trapped in the geodes .

A second cooling then takes place  gas this time, from 400 °C, depending on the composition of the gas trapped in the cavity so formed. This gas then passes into aqueous phase and hydrothermal phenomena can begin. The first ( and sometimes only ) crystallization may therefore be entirely internal to the geode , without external input . Then, the transformations can continue millions of years , it is also the paradoxical aspect of geodes that being a part of the minerals with rare live yet mineral inputs that may exist millions of years while constantly evolving while keeping a permanent form of their original heritage. (wikipedia)

For volcanophilatélistes , there is little stamps on the theme of the geode ... a stamp of French Territory of the Afars and Issas (Somali coast, Djibouti ) of 1971  shown a volcanic geode.

 

Sources :

- Géowiki - Okénite

- Mindat - Okénite -  fiche & photos

- Webmineral - Okénite

- Mineralogical magazine - Zeolites and associated secondary minerals in the Deccan traps of western India - by R.N.Sukheswala & al. - link