Zeolites of The World – A Book by Rudy W. Tschernich


Zeolites of The World – A book by Rudy W. Tschernich

Zeolites Defined
Origin of the Name
The name zeolite was created in 1756, by Freiherr Axel Fredrick Cronstedt, a Swedish mineralogist, from the Greek word “zein” (to boil) and “Iithos” (stone), to describe a group of silicate minerals that expel water when heated and seem to boil and puff up in a borax bead, followed by melting to a white glass. The in in “zein” was dropped, an 0 was inserted, and lithos was changed to lite and added to ze-o- to create the word zeolite.

Early History
Zeolites were known by many local names in the eighteenth century. Most of the research in zeolite mineralogy was done, at that time, in Germany, Italy, France, and Great Britain. Because of differences in language, nationality, and obscurity of some of the publications, no uniform system of nomenclature had been developed. There was no governing body, like the International Mineralogical Association (IMA) of today, to determine if a proposed name and description was the same as another species.

The general acceptance of the mineral names used in Traite de Mineralogie by HaUy (1801) formed a basis for more consistent nomenclature. Haily is given credit for naming stilbite, analcime, harmotome, and laumontite although he only popularized and more completely described mineral names already used in obscure publications. These few zeolites formed the basis of the zeolite group, which has 48 species accepted by the IMA today (see Zeolite Time Line, p 9).

Mineralogical testing in the eighteenth and nineteenth centuries consisted of blow pipe reactions and determinations of physical properties (hardness, cleavage, density, streak), morphology, optical properties, and chemical properties. Chemical analysis was performed by wet bulk analysis in which a considerable amount of mineral sample was powdered, dissolved in acids, and precipitated as oxides, dried, and weighed. Pure samples produced accurate results, but chemically zoned minerals, intergrown minerals (several zeolites or other species), and unknown elements produced erroneous results. For instance, the zeolite wellsite was named because it contained a composition (determined by bulk wet chemical analysis) between harmotome and phillipsite. Much later, these “wellsite” crystals were found to be phillipsite in the core and harmotome at the rim; therefore, the name “wellsite” was not needed. A continuous series in chemical composition was found between the fibrous zeolites, natrolite, mesolite, and scolecite because intergrown samples were powdered and analyzed. Much later, gaps in chemical composition were found between these species. Pollucite was first described as a sodium and potassium silicate because cesium had not been discovered yet.

Mineralogists of the nineteenth century believed that the internal crystal structure of minerals. might be understood by the extensive study of crystal morphology. Victor Goldschmidt brought together the morphological studies from this period in his eighteen-volume Atlas der Kristallformen (1913-1923).

Dana (1898) defined zeolites as a family of hydrous Silicates, closely related in composition, in conditions of formation, and hence, in method of occurrence. They are silicates of aluminum, chiefly with sodium and calcium, also rarely, barium and strontium.

With the introduction of X-ray diffraction in the 19205, the internal structure of minerals could finally be examined. Many old zeolite species, separated by morphology and chemical properties, were found to be identical and were discredited (see zeolite synonyms). The aluminosilicate structure of analcime was the first zeolite structure to be determined (Taylor, 1930). At the same time, Hey (1930) recognized that all zeolites had hydrated aluminosilicate frameworks with loosely bonded alkali and alkaline earth cations and these provided the first chemical criteria for distinguishing zeolites from other minerals.

Chemical Criteria for Zeolites
Hey (1930) showed that a mineral has a three-dimensional framework structure when O/(Si+ AI) = Z. Framework structures are found in zeolites, feldspars, feldspathoids, scapolites, quartz minerals, and danburite. The relationship between the exchangeable cations and aluminum content in framework aluminosilicates is always (Ca +Mg+Ba+ NaZ+ KZ)/AI = 1. Zeolites differ from all the other framework silicates by the presence of water molecules.
The general formula for a zeolite mineral is
(Ca,NaZ,KZ,Ba,Sr,Mg,CsZ,LiZ)a[AlaSin_a0Zn] •xHZO
The part in the square brackets represents the framework atoms and the part outside the brackets, represents the exchangeable cations plus water molecules. The variables (a, n, and x) depend on the composition of each species.

In some synthetic zeolites, aluminum and silicon cations have been replaced by beryllium, iron, gallium, germanium, or phosphate ions (Gottardi and Galli, 1985). In synthetic zeolites, all alkali, alkaline-earth, rare earth elements, and organic complexes, such as tetramethylammonium, can occur as exchangeable cations (Gottardi and Galli, 1985).

Relationship of Framework Minerals
Zeolites are just one of the several groups of minerals with a framework structure. Quartz minerals are composed of tetrahedra with four oxygen ions 0-2 at each of the four corners and a silicon ion Si+4 in the center. To electrically balance the mineral, each tetrahedra is linked by sharing the two negative charges of oxygen between adjoining tetrahedra.

In the feldspar, feldspathoid, scapolite, and zeolite groups, aluminum ions AI+3 substitute for silicon in the center of some of the tetrahedra and produce an electrical imbalance in the framework that must be counterbalanced by adding positively charged cations (Ca, K, Na, Li, Mg, Sr, Ba, Cs) to produce an electrically neutral stable mineral.

The feldspars have a compact aluminosilicate structure with sodium, calcium, potassium, and barium ions in relatively small cavities completely surrounded by oxygens of the framework. The cations and framework in feldspars are so strongly interdependent that cations cannot be easily exchanged.

The feldspathoid minerals (leucite, nepheline, sodalite, and others) have a somewhat more open aluminosilicate framework with channels running through the structure. Ions in feldspathoids can be exchanged and can accommodate small molecules.

The aluminosilicate frameworks of zeolites are similar to feldspathoids but are still more open, with large channels and interconnected voids occupied by loosely bound cations and water molecules that can be removed and replaced without disrupting the framework bonds. In some zeolites, cation exchange or dehydration may produce structural changes in the framework.

Zeolite Framework Analogy
The internal structure of a zeolite can be compared to that of a house. The walls, floors, and ceiling of the house are the silicon-aluminum-oxygen tetrahedral framework of a zeolite. The walls can be arranged in different ways to form interconnected hallways (channels) and rooms (cages). The framework or walls commonly meet at 90° but some can be inclined by a few degrees. There is a lot of open space in a house and in the structure of a zeolite. Some of the rooms (cages) and hallways (channels) are of different sizes. Furniture (exchangeable cations) and people (water molecules) can be moved about within the rooms, be replaced, or be totally removed without collapse of the house or framework. The furniture can be neatly arranged like ordered cations or placed at random like disordered cations.

A Definition of a Zeolite Mineral
A zeolite mineral is principally an aluminosilicate with a three-dimensional framework structure composed of Al04 and Si04 tetrahedra linked to each other by sharing all of the oxygens to form interconnected cages and channels that contain mobile water molecules and alkali (sodium, potassium, lithium, and cesium) and/or alkaline earth (calcium, strontium, barium, and magnesium) cations. Subordinate amounts of other elements such as phosphorus and beryllium can replace silicon and aluminum, respectively, in amounts up to 50 percent. In many zeOlites, the water molecules and exchangeable cations can be removed or exchanged without affecting the structure, while other zeolites undergo small distortions in their framework. Water is lost between 250° to 400″ C and is reversibly reabsorbed at room temperature. Ion exchange is possible at temperatures below 100° C. Higher polyvalent ions, such as rare earths, are readily introduced by cation exchange.

Heulandite (Hew-Iand-ite)
(Ca,Na2,K2,Sr,Ba,Mg)S[A1SSi2S0n] 020-2492°, Z = 1

Named in 1822, by H. J. Brooke, in honor of John Henry Heuland (1778-1856), a British mineral collector and dealer.

Type Locality: None

Obsolete Synonyms: beaumontite, c1inoptilolite, euzeolite, euzeolith, lincolnine, lincolnite, metaheulandite, stilbite anamorphique, stilbite octoduodecimal

Nomenclature: Heulandite was defined as the member of the heulandite-clinoptilolite series where the sum of calcium, strontium, and barium is greater than the sum of sodium and potassium (Mason and Sand, 1960). Boles (1972) defined heulandite as the member with a Si/Al ratio less than 4. Mumpton (1960) took a different approach and defined heulandite as the member that had the framework that would collapse if heated overnight at 450° C, and clinoptilolite as the member with the framework that was intact after heating. If each of these approaches is applied to some samples, there is a lack of consistency between them.

Heulandite and c1inoptilolite have the same aluminosilicate framework with only slight differences in those with high-silica or divalent cations. A continuous series between the calciumdominant low-silica heulandite and potassium-sodium high-silica c1inoptilolite has been well established. Although both heulandite and clinoptilolite are recognized by the International Mineralogical Association (IMA), there is no reason to maintain two separate species. Since heulandite has priority (it was named first), the name clinoptilolite has been discontinued as a species name in this book.

Variations in chemistry or order-disorder in the framework should be handled by using optional descriptive modifiers (such as Ca-rich, calcium, calcian, sodian, potassium, Sr-bearing, disordered, ordered, silica-rich) rather than creating new species.

Type of Structure: The framework consists of 4-4-1-1 units that form chains linked by translation to form sheets (Gottardi and Galli, 1985). The aluminum and silicon in the framework has some ordering. One of the five tetrahedral sites contains 50 percent of the aluminum, two sites contain nearly zero aluminum, and the other two sites contain the remainder of the aluminum (Gottardi and Galli, 1985). As the silicon content increases in some crystals, only 30 to 40 percent of the aluminum is found in the main tetrahedral site. Low-silica heulandite contains two additional sites containing water molecules that are not present in silica-rich heulandite. There are two types of channels parallel to the c-axis and a third type of channel parallel to the a-axis (Gottardi and Galli, 1985).

All of the exchangeable cations are found in two sites within the channels parallel to the c-axis. One of these sites contains all of the calcium while the other site contains all the other cations (sodium, potassium, magnesium, strontium) (Alberti and Vezzalini, 1983).

Heulandite has a wide range in exchangeable cations and aluminum and silicon in the framework. Most large heulandite crystals are calcium-dominant with a low silica content. The silicarich heulandite (= clinoptilolite) is commonly dominant in potassium or sodium. Strontium and magnesium are present in small amounts and rarely are the dominant cations. Barium is present in small amounts. Water content varies over a wide range. TSi = Si/(Si+AI) = 0.71 to 0.83; Si/AI = 2.84 to 4.99

The general formula for heulandite can vary from the common low-silica variety (Ca,Na2,K2>Mg)9[AI~i270n]o24H20, to the high-silica variety (Na2,K2,Ca,Mg)6[Al6Si300n] o20H20 .

Heulandite is one of the most common zeolites. It is found widely in volcanic rocks, metamorphic rocks, pegmatites, altered volcanic tuff deposits, and in deep-sea sediments. Due to the extremely small size of silica-rich heulandite (= clinoptilolite) crystals in volcanic tuff and deepsea sediments, only a few of those extensive occurrences are described.

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