Stilbite and Heulandite by Barry Taylor

This beautiful Zeolite is also known as Wheat Sheaf Ore due to its distinctive crystal shape when twinned, looking like the stooks of wheat,
fresh in the fields when hand gathered. Stlibite is commonly found ...

as either white or light pink crystals, and occurs in amygdaloidal vugs or
cavities within a basalt matrics. Amygdaloidal refers to the almond shape of many voids or pockets within lava. Stilbite can also occur in Hydrothermal Veins and even Hot Spring deposits as the elements that go together to form this mineral exist in this environment.

Crystal form
Stilbite is a Hydrated Sodium Calcium Aluminium Silicate that crystallises in the Monoclinic System. It is usually white but sometimes is also found as crystals of yellow, brown, orange, red, green or Black colour, this variable hue is due to minute inclusions of other minor elements that impart a strong colour.

This Zeolite has a distinctive crystal form in most examples but it was not until 1818 that it was actually recognised this as a separate
species. This mineral has been named after John Henry Heuland the British Mineral collector and dealer. As it was he who first identified it
as a different mineral from Stilbite with which it is often found.

Crystal form
Heulandite is a hydrated Calcium Sodium Aluminium Silicate and is the Calcium rich member of this Zeolite group. This Zeolite crystalises also in the monoclinic system, the crystals have a coffin shape and both minerals can occur together hence the confusion, both often have the same colour. When looking at Zeolites many are just white and therefore identification is much more complex as the environment of formation is exactly the same; both these minerals are known as tecto-silicates.

A rare related Zeolite this mineral is also a hydrated Sodium Silicate, it crystallises in the triclinic crystal system and it actually has chemical similarities in its mineral structure to Mordenite. The crystals form as clusters of platy crystals and have a similar colour to the other Zeolites, the name means on top of and that it often how it is found, on the top of Stlibite.

Crystal form
Like the crystals of Stilbite as mentioned above, it often occur as distinct platy crystals. This mineral much like the other zeolites is known as a late stage mineral and therefore forms on top of earlier minerals, they form at a lower temperature within crystal vugs often on top of Chalcedony.

Zeolite Crystal uses
These crystals are used in industry as what are called chemical sieves; this is a very useful feature of these Zeolites that is employed in many of our homes. In the case of Stilbite they can contain either Sodium or Calcium in a loosely bonded chemical framework. This means that if you pass water rich in Calcium through the Sodium rich version of the mineral, the Sodium exchanges for the Calcium and then softens the water. At night in a reverse system the Calcium is then flushed out and replaced by Sodium in the form of the salt that you put in the water softener. Heaulandite is used in a similar way but instead it is used in a similar way in petrochemical refinement plants.

Technical information
Stilbite as stated above is a hydrous calcium sodium and aluminium silicate and has both Sodium and Calcium rich members, it crystallises in the monoclinic system as well as the triclinic and orthorhombic systems, it has a hardness of 3.5 to 4 on the Mohs scale.
Heulandite is similar but more hydrated and the Calcium can also be replaced by Strontium, it is monoclinic and it also has Potassium and Barium rich members.

This group of Zeolites as you can see is a very complex set of minerals that are difficult to define, even by experts without the use of
special laboratory equipment. Only the basic crystal shape and comparison with other examples will help, combined with knowledge of the location can aid with identification of the crystals.

The Tertiary Deccan Traps of India are the most prolific source followed by Iceland, the Isle of Skye, Nova Scotia New Jersey, Carolina, Australia and Italy.

Written by
Barry Taylor

To see a list of mineral, crystal, gemstone, fossil shows/events both UK and worldwide visit our show page, if you organise or know of a show please get in touch to list it here.

Copyright © 2021 Rock n Gem Magazine. All Rights Reserved.
Hosting and website design by EarthlygemsIT