by Barry Taylor
My all time favourite minerals are the spectacular Zeolites from the Deccan Plateau region in India, this is a flood Basalt deposit that formed over several million years.
This is one of the largest volcanic features on earth and covers 163,000 square miles to a depth of 6,500 ft, the area covered by lava incorporates eight seperate states of India and covers a large part of the country.
This amazing igneous formation was deposited at the same time that the Dinosaurs died out 65 million years ago, the outpourings of lava are considered to have been a contributing factor in the mass extinction, along with the asteroid impact at Chicxulub. Indeed the impact could well have triggered the volcanic outpourings that conveniently occurred around the same time on the opposite sides of the earth. Also it is possible that both these events are thought to have happened in a similar coastal submarine environment with different consequences for the planet.
Zeolites together with other associated minerals such as the common mineral Apophyllite form in vugs and cavities in the lavas, they are created by circulating ground waters that act on minerals within the lavas, leaching them out. Mineral vugs in the lavas are not found everywhere but are most commonly found in localities around Bombay, Baroda, Poona and Nasik.
With regards to the Decan Plateau Traps, in 1969 a zonal formation for minerals was identified, this has lately been questioned by some mineralogists. This zonal identity was based on the most abundant minerals in each region, the mineral zones are related to the depositional height of different lava flows, running from the low coastal plains to the higher inland hills as follows.
Laumontite and Mordenite zone this coastal zone is at the bottom, these minerals in particular Laumontite are delicate and very brittle, laumontite also degrades on exposure to air. This group of minerals occurs together with Heulandite, Stilbite, Scolecite and Apophyllite, together they are abundent in the coastal tract in Nasik, Baroda and Panvel.
Scolecite and Mesolite zone is next and occurs in the higher hilly area around Lonavla and south east of Nasik, with Bombay on the boundry between the two zones, in this zone the minerals are most often acicular, forming banded masses of radiating minerals and tufts of smaller crystals like Scolecite, Mesolite and Natrolite. These are found together with the other zeolites, Heulandite, Stilbite and Apophyllite as well as other Silica rich minerals like quartz and Chacedony.
Heuladite and Stilbite zone, the final zone in the higher hills, here these minerals are usually found as cream and pink crystals, red examples when found often fade gradually on exposure to air. Also found in this zone are Chabazite, Mordenite and Apophyllite this zone assemblage is typical for the Poona area in the Syhadree Mountains.
Associated Zeolite Minerals include the wonderful Apophyllite which has a wide variety of crystal forms from blocky to pyramidal with crystals of various colours as well as the clear lustrous columns in beautiful examples. Chalcedony is often cellular in form with complex stringers. Quartz crystals as well as Calcite crystals form in association with zeolites in this zone. Also in particular Chalcedony often acts as a base mineral usually grey, cream, black or blue colour upon which the later zeolites occur and stand out asthetically. Another associated mineral is Prehnite, this occurs as blocky crystals and also forms as an epimorph after Laumontite with hollow greenish columns remaining. The less common zeolites found are Chabazite, Epistilbite and Thompsonite, with Gyrolite and Levyne they make up only 2% of the minerals found. Spectacular contrasting Babingtonite forms as small black crystals scattered on top of white Zeolites. Secondary Chlorite minerals also occur often adding Green and Red colour to the zeolites as well as some background.