Zeolite

A Form of Zeolite Mineral
A Form of Zeolite Mineral

Zeolite constitute a group of hydrated microporous aluminosilicate minerals (mainly Na, K, and Ca) whose members have similar characteristics in terms of chemical composition and basic crystalline structure.

They are found as natural products of pyroclastic rocks in volcanic tuffs, but they are also synthetically prepared.

Zeolites belong to the larger group of tetrosilicate minerals, and are called molecular sieves’, because several substances of a certain size can pass through the channels and cavities of their lattice – from their porosity.

Natural Zeolites can absorb up to 30% of their dry weight various gases (eg ammonia and nitrogen), while they can absorb more than 70% by weight of water and up to 90% of some hydrocarbons.

The general chemical formula describing a Zeolite is M2/nΟΑl2O3·xSiO2·yH2O where:

  • M: the cation with valence n
  • x: integer between 2 and 10
  • y: integer between 2 and 7

The ratio M2/nΟ: Αl2O3 is equal to 1, while the ratio (Si + Al)/O is always equal to 2.

CONTENTS

DOCUMENTS

Heulandite Mineral from zeolithic tuffs in Canada

Heulandite 5.7 cm from Cape Split, Canada
Heulandite 5.7 cm from Cape Split, Canada

Etymology

The name Zeolite is a neologism, which has an ‘aristocratic’ origin, since it was given to the mineral by the Swedish Baron Axel Fredrik Cronstedt (23 December 1722 – 19 August 1765) in 1756. 

It is a composite word of two Ancient Greek, of ‘ζέω‘ (zeo = boil) and ‘λίθος‘ (lithos = stone). The first part, the word ‘ζέω‘, has a Proto-Indo European origin, as its core is the ‘*yes-‘ root (Sanskrit: यसति) and means ‘to froth up, foam‘.

The second component, the word ‘λίθος‘, is of unknown origin and etymology but very well understood meaning, and is therefore highly prized by geological science as it is found in a number of geological terms.

The explanation, why Zeolite was called ‘boiling stone‘, is given shortly after.

Brief History of Zeolites

Prehistory & Antiquity

Since prehistoric times man has used zeolithic rocks to make tools and construct buildings. Sometimes it even seems to have been inhabited in them, as the Cimmerians are supposed to have been living in an underground city, near the Ancient Greek city of Cymae (Kyme) in Magna Graecia (South Italy).

Everything shows that the Ancient Greeks did not follow the example of their neighbors. From the 7th century BC used the rocks that contained Zeolite as basic material for the overground construction of houses as well as any other kind of buildings.

The Romans continued to use this material, and even used it as a means of purifying water, in the famous aqueducts they built. Still more, the Romans created a very durable type of cement, consisting of pozzolan and powdered zeolithic rock.

On the other side of the world, on the American continent and especially in Pre-Colombian Mexico, the indigenous Zapotec, already in 2000 BC used zeolithic rocks to create glyphes, buildings and other constructions. Their extraordinary art brings us up to our current day, as some of their creations are preserved in the ancient city of Mitla, located in the province of Oaxaca.

On the other side, of the other side of the world, in Japan, began in the 5th century AD exploited zeolithic rocks for the construction of buildings in the ancient city of Utsunomiya, the capital of Tochigi province, Honshu Island.

In contrast, the opposite Chinese preferred to use them as drugs: in China, for centuries, Zeolite formulations were used for healing purposes.

18th Century

Since ancient times, it only took a few centuries until the 18th and the above-mentioned Axel Fredrik Cronstedt came, bringing the Zeolites to the light of science. But who was this gentleman?

Well, Axel Fredrik Cronstedt in addition to Baron was also an excellent chemist and mineralogist, that among others, he was the one who discovered in 1751 and Nickel.

In 1756 two samples of unknown minerals from Iceland and Sweden came into its possession. In some phase of his work with this material, Cronstedt attempted to melt the crystals – Stilvitis as it is supposed. As the process progressed, he noticed that the heated crystals seemed to steam, giving the impression that they were boiling.

With this in mind, Axel Fredrik Cronstedt has decided to name Zeolite the material, thanks to his extensive classical education.

However, the first undisputed proof of the recording of Zeolite – the Chabazite – is given by Bosch D ‘Antic in 1792.

19th Century

About 50 years later, another researcher dealing with the Zeolites was the French mineralogist Augustine Alexis Damour (19 July 1808 – 22 September 1902). Distributing his interest between his science and the study of Prehistory, in 1840, makes known that Zeolite crystals can reversibly render water without altering their structure.

By the end of the century, his compatriot and colleague mineralogist but also and chemist Charles Friedel (12 March 1832 – 20 April 1899) demonstrated that space left empty in a Zeolite after dehydration can be covered by other molecules such as ammonia, carbon dioxide, hydrogen, hydrogen sulfide and ethanol.

Modern Era

A pioneer in modern research about the Zeolites, was the New Zealand chemist Richard Maling Barrer (16 June 1910 – 12 September 1996). His research work focused on membrane permeability, desorption, and composition of Zeolites. In his work is offered the first classification of the Zeolites, and in his name offers Zeolite Barrerite its own name.

Since then, the Zeolites have been the subject of exploration and exploitation by industry. The ability to create synthetic Zeolites has promoted them in a wide range of applications, spreading their use even more.

The number of synthetic zeolites now stands at about 160, while the number of natural ones reaches 67.

Zapotec Glyphs at Monte Albán
Zapotec Glyphs at Monte Albán

Structure of Zeolites

The basic stuctural element of Zeolites is tetrahedron [(Si,Al)O4]-4. The tetrahedron is composed of 1 aluminum or silicon atom surrounded by 4 oxygen atoms symmetrically positioned on the tetrahedron. The atomic ratio of oxygen to silicon atoms and aluminum equals two.

Zeolite Primary Building Unit - Tetrahedron

From the union of the tetrahedrons, which are also called primary building units, polyhedral secondary building units are formed that form single, double or branched rings. Each ring contains from 4 to 16 tetrahedrons, while the junction of the rings results in the formation of chains and cells. The rings, chains, and cells ultimately give the separate form of the Zeolite structure.

Characteristic of the structure of all Zeolites is the existence of large empty spaces (thethree dimensional cages and channels), the volume of which can reach up to 50% of the dehydrated mineral, and their size ranges between 0.22 and 1.18 nm.

Physical & Chemical Properties

Colour

Zeolite is found in different colors but can be colorless or transparent. White, colorless or transparent is when it is in pure form, while greenish to slightly brownish-reddish is when it is mixed with impurities.

Density

The density of Zeolite varies between 2 to 2.3 g/cm3, while those containing abundant barium (Ba) have a density of between 2.5 and 2.8 g/cm3.

Bulk Specific Gravity

Zeolite has a bulk specific gravity ranging between 0.80 and 0.90 g/cm3.

Refractive Indices

Optically, the refractive indices of its minerals are ranging between 1.47 and 1.52.

Containing Humidity & Water Absorption

In terms of humidity, Zeolite contains less than 3.9%, and in terms of water absorption capacity, it ranges from 45 to 75 mL /100 g.

Ion Exchange Capacity

Previously mentioned the existence of cages and channels. Within these spaces, the water molecules, as well as the weakly grid cations Na, K, and Ca, circulate freely. Cations are exchanged with great ease because of the great ion-exchange capacity that Zeolite has. In particular, the Cation Exchange Capacity (C.E.C.) of mineral is usually between 200 and 300 cmol kg-1, but can be up to 400 cmol kg-1.

The ion exchange capacity of Zeolites is affected by many factors. The most important of these are:

  • The characteristics of the structure of Zeolite
  • The density of the anionic charge in the structural framework
  • The shape and dimensions of the new cation
  • The solvent
  • The pH of the solution
  • The temperature of the solution
  • The concentration of electrolyte in the solution
  • The degree of substitution of Si ions from Al ions

The following table gives the Cation Exchange Capacity (C.E.C.) of some Zeolites.

Zeolite Mineral

C.E.C. (meq/g)

Analcime

3.6 – 5.3

Laumontite

3.8 – 4.3

Erionite

2.7 – 3.4

Faujasite

3.0 – 3.4

Phillipsite

2.9 – 5.6

Chabazite

2.5 – 4.7

Clinoptilolite

2.0 – 2.6

Heulandite

2.2 – 2.5

Mordenite

2.0 – 2.4

Ferrierite

2.1 – 2.3

The ion-exchange capacity that characterizes Zeolites was the first property of these minerals, which was investigated in depth.

Adsorption Selectivity

Within the Zeolites pores, various molecules and ions of various sizes are trapped. Molecular filtration capability depends on their crystal structure and separation capacity based on size, shape, or polarity, giving Zeolite another remarkable feature, that of selectivity adsorption.

This phenomenon is due to its characterization as a ‘molecular sieve’, which is mainly used for the separation of gas mixtures.

Ability to Catalyze

The ability to catalyze Zeolites is largely due to the acidic regions inherently scattered in various parts of their crystalline lattice. Also, the size of both their surface resources and their internal cavities where the reactions take place affects the catalysis capacity. Still any Zeolite can influence the selectivity of a reaction as follows:

  • Based on the selectivity of the reactant
  • Based on the selectivity of the product
  • On the basis of the selectivity of the transition state
  • On the basis of molecular traffic control

Thermal Stability

The thermal stability of zeolites reaches 1000 °C.

Structural Stability

Each Zeolite presents – depending on the type – excellent resistance to acid or alkaline environments as well as to radioactivity (as a radionuclide absorber – mainly Sr and Cs).

Natrolite 5.0 cm from Cape Split, Canada
Natrolite 5.0 cm from Cape Split, Canada

Classification of Zeolites

Natural Zeolites are classificate in 8 families, with 7 of them having a name, but for the eighth is still looking for a godfather. And while is following a detailed record of the seven baptized families, it is noted here that the latter and unbaptized include the mineral Zeolite Dachiardite and the Tetranatrolite.

1.Analcime

Zeolite MineralShape
Analcime

 

 

 

Cubic/Tetrahedral

Pollucite
Wairakite
Bellbergite
Bikitaite
Boggsite
Brewsterite

2.Chabazite

Zeolite MineralShape
Chabazite

 

 

 

 

Rhombohedral

Willhendersonite
Cowlesite
Dachiardite
Edingtonite
Epistilbite
Erionite
Faujasite
Ferrierite
Herschelite

3.Gismondine

Zeolite MineralShape
Amicite

 

 

 

Monoclinic/Orthogonall

Garronite
Gismondine
Gmelinite
Gobbinsite
Gonnardite
Goosecreekite

4.Harmotome

Zeolite MineralShape
HarmotomeMonoclinic
Phillipsite
Wellsite

5.Heulandite

Zeolite MineralShape
Clinoptilolite

 

 

 

 

Monoclinic/Orthogonall

Heulandite
Laumontite
Levyne
Mazzite
Merlinoite
Montesommaite
Mordenite
Maricopaite

6.Natrolite

Zeolite MineralShape
Mesolite

 

 

 

Orthogonal/Tetrahedral

Natrolite
Scolecite
Offretite
Paranatrolite
Paulingite
Perlialite

7.Stilbite

Zeolite MineralShape
Barrerite

 

 

Monoclinic

Stilbite
Stellerite
Thomsonite
Tschernichite
Yugawaralite

The most widely used natural Zeolites – those who have found scope – are just 9. The following table is listed along with their chemical formula.

Analcime Zeolite 5.8 cm from Five Islands, Canada
Analcime Zeolite 5.8 cm from Five Islands, Canada

Wider Used Zeolites

Zeolite Mineral

Chemical Formula

Analcime

Na[AlSi2O6]H2O

Laumontite

(Ca4[Al8Si16)O48]16H2O)

Erionite

(K2,Ca,Mg,Na)4,5[Al8Si27O72]28H2O

Faujasite

Na13Ca12Mg11[Al59Si133O384]235H2O

Phillipsite

(0,5Ca,Na,K)6[Al6Si10O32]12H2O

Chabazite

Ca2[Al4Si8O24]12H2O

Clinoptilolite

Na6[Al6Si30O72]20H2O

Heulandite

Ca4[Al8Si28O72]24H2O

Mordenite

Na8[Al8Si40O96]28H2O

Synthetic Zeolites

The first synthetic Zeolite was created in 1948 by Richard Maling Barrer, having as a natural analogue the Mordenite. A little later, but the same year follows Milton, who composes Zeolite A.

Between 1954 and the early 1980s, research and production of synthetic Zeolites was ejected.Since then, more than 160 separate Zeolite structures have been registered, as has been previously mentioned, with the prospect of more.

Naming & Dissemination of Types

For the naming each of the framework types (natural and synthetic) of the crystalline structures, the International Zeolite Association (I.Z.A.) has established a code name. The name is attributed to the use of three letters resulting from the ‘kind of material’. Thus, as example are reffered the code name FAU is derived from the Faujasite mineral, and MFI, which is derived from zeolite ZSM-5 (Zeolite Socony Mobil-Five).

The most commonly used types of synthetic Zeolites are:

  • Zeolite A
  • Zeolite X
  • Zeolite B
  • Zeolite Y
  • Zeolite SZM-5

Categories of Synthetic Zeolites

Synthetic Zeolites based on their silicon content are distinguished in:

  • Low Silica Zeolite (Si/Al ~ 1-1.5)Low Silica Zeolite
  • Intermediate Silica Zeolite (Si/Al = 2-5)
  • High Silica Zeolite (Si/Al> 10)

Silicon/Aluminium ratio categorization reflects specific properties for each separate Synthetic Zeolite. Specifically, the small Si/Al ratio:

  • Increases the ion exchange capacity of the material
  • It gives greater adsorption capacity for polar molecules

Conversely, the high Si/Al ratio:

  • Increases the hydrothermal stability of the material
  • Gives greater catalytic activity
  • Increases the hydrophobicity of the material as a sorbent

By controlling the manufacturing process, each synthetic Zeolite can alter these characteristics to suit the use of the material.

Another classification, for every synthetic Zeolite, is based on the size of its porous. According to this, the synthetic Zeolites distinguish as:

  • Microporous Zeolites (8-ring porous)
  • Mesoporous Zeolites (10-ring porous)
  • Macroporous Zeolites (12-ring porous)
  • Hypermacroporous Zeolites (14+ ring porous)

The classification of porosity reflects a different property field, that of catalysis. Indeed, porous size affects the available space of Zeolite and affects the molecules that can be adsorbed and desorbed.

Zeolite - Clinoptilolite 5-8 mm
Zeolite - Clinoptilolite 5-8 mm

Zeolite, Deposits & World Production

Although many decades has passed since the end of the Cold War, most countries and most of the companies that exploit Zeolite do not provide data or give inaccurate data on the amount of deposits and production.

Therefore, only approximate – and of relatively little value – can be used to calculate the actual world production in Zeolite. And in order to give a picture of the prevailing confusion, suffice it to be mentioned, that in many considerations Zeolithic minerals are deposited, in which zeolite is contained in moderate to minimal concentrations.

Zeolite - Deposits & Production in Asia

Japan: In 1949, a field of Zeolite was discovered for the first time, with a large Clinoptilolite content, in solid green tuffs as well as Mordenite deposits. Today the first is excavated from Oshyamambe, Futatsui, Itaya, Nishiaizu and Shimane, while the second from Itado, Maji, Iizaka, Shiroise and Itaoroshi.

Indonesia: It has Zeolite deposits in Sumatra, Panjung and Mount Rataiand, and Java. The minerals consist of Clinoptilolite and Mordenite, while the Clinoptilolite located on Mount Rataiand is of high quality.

Jordan: It has deposits mainly containing Phillipsite, in the Rashadieh, Tel Remah, Tel Hassan and Al Aritayn regions.

China: It is the world’s largest producer. Zeolite deposits from various areas are exploited by mining companies. The ores contain Clinoptilolite, Mordenite or Zeolite powder. In Tianjin and Tsagaantsav areas, the Clinoptilolite contained in ores reaches 80 to 96%.

South Korea: An important producer country. It has Zeolite deposits in Tonghai and the Guryongo area. The ore consists of Clinoptilolite and Mordenite.

Turkey: In the Manisa and Gördes areas there are Zeolite deposits containing 85 to 95% of Clinoptilolite.

Zeolite - Deposits & Production in Australia

Australia: Deposits of Clinoptilolite exist in the Werris Creek, Quirindi and Castle Mountain areas in the state of New South Wales. Also in the State of Queensland, in Willows.

Zeolite - Deposits & Production in Africa

South Africa: In Mount Lebombo there are deposits containing 85 to 90% of Zeolite (Clinoptilolite and Mordenite)

Zeolite - Deposits & Production in North America

United States: Natural Zeolites are located in Idaho, Bowie of Arizona, California, Cuchillo Negro of New Mexico, Oregon, Nevada and Texas. Apart from Arizona, where the ore consists of Chabazite, in all other areas it consists of a Clinoptilolite.

Cuba: It has large Zeolite deposites. The ore consists of Clinoptilolite (Caimanes and Moa), Heulandite (Carolinas), Mordenite (Bueycito and Palenque) and Analcime. The zeolithic minerals have high Ca and low Na content.

Mexico: Zeolithic deposits exist in the Chihuahua, Cuitzeo, Puebla (Clinoptilolite and Heulandite), San Luis Potosí (Clinoptilolite and Heulandite) and Sonora regions.

Zeolite - Deposits & Production in Europe

Bulgaria: Zeolithic tuffs are located in East Rhodope, Beli Plasti and Jelezni Vrata. In the first there are sufficient deposits of Clinoptilolite, in the second zeolithic rocks and tuffs that are used for the production of cement.

Germany: In the Kaiserstuhl area there are deposits containing about 45% of Zeolite.

Greece: High quality zeolithic deposits exist in the Evros area, in the village of Petrota (Clinoptilolite with Heulandite).

Spain: Zeolite deposits exist in Almeria (Mordenite) and in the province of Madrid.

Hungary: Zeolite deposits exist in Mad-Suba, Ratka, Bodrogkereszturrr and Mad-Vasut. The first three include Clinoptilolite, while the last Mordenite.

Ukraine: Zeolithic tuffs are located in the Sokyrnytske district of Hust, in the region of Zakarpattia. The base ore is Clinoptilolite.

World Production of Zeolite (2015)

Considering the above mentioned, the table below shows the estimate for world production of Zeolite in 2015.

CountryProduction (in t)
China<2.200.000
South Korea250.000
United States75.100
Turkey70.000
New Zealand65.000
Cuba43.000
Jordan13.000
Mexico<2.500
Indonesia1.500

Infographic of estimate Global Zeolite Production 2015:

Infographic of Global Zeolite Production 2015

Countries that produce natural Zeolites but for which there were insufficient data to produce relatively reliable estimates of production include Argentina, Armenia, Australia, Bulgaria, Canada, Georgia, Germany, Greece, Hungary, Iran, Slovakia, Slovenia, South Africa, Spain as well as Ukraine.

Global Production of Synthetic Zeolite

As a statistical outline, world production of synthetic Zeolite in the year 2018 is estimated at 12,000 t, yielding $ 5 billion.

Zeolite - Stilbite & Apophyllite Crystal Cluster
Zeolite - Stilbite & Apophyllite Crystal Cluster

Zeolite & Its Use

Zeolite, both in its natural form and as a synthetic, is exploited in various fields. Moreover, both forms of Zeolite are complementary to various applications, because:

  • Where, in a particular application, is the uniformity and purity of the material, then the synthetic Zeolite is used
  • On the contrary, where the requirement is not the uniformity and purity of the material, then the natural Zeolite is used

Synthetic Zeolite is utilized in high cost applications, while natural Zeolite is used in low-cost applications.

Zeolite in Construction

Zeolite is used as a component in cement production as a filler in the manufacture of compressed boards, as well as in the manufacture of high natural and chemical strength, lightweight bricks.The advantages of these materials are their durability and strength, both in abrasion and friction.

In addition, Zeolite is used – as it has for 2,000 years – as a dimension stone in building industry. Zeolite buildings have a remarkable thermal insulation capacity, and their use in stall construction reduces ammonia and odor of space.

Zeolite in Industry

In order to replace phosphate compounds – which are largely responsible for contamination and degradation of the aquatic environment – Zeolites are used by the detergent industry.

Another field of application is the purification of industrial wastewater as well as the use of Zeolites as filters for the retention of various harmful substances and for the carbon dioxide capture.

In the petroleum industry, Zeolite is used, in addition to the cleaning of petrochemicals, as a catalyst both in the oil spill phase and in the production of high octane fuels.

In the gas industry, Zeolite is used in both production and sanitation.

The proven increase in fabric life and the significant reduction in allergies that they can potentially cause has made it necessary for Zeolites to be used by the textile industry.

In the paper industry, Zeolite is used as a mass additive.

Zeolite in Environmental Applications

Zeolite is widely used as a filter for sanitizing drinking water. This is because its efficacy has been proven to both neutralize pathogenic microorganisms and to block ammonium and heavy metal ions.

The role of Zeolites is also valuable in the capture and removal of radioactive isotopes from nuclear waste.

Zeolite is still used to clean urban waste water, resulting in a significant increase in quality characteristics.

Zeolite in Agricultural Applications

Cultivations: Zeolite has a lot of applications in the agricultural sector. Zeolite has a lot of applications in the agricultural sector. With the unexpected from the ‘Kalliergeia’ editorial team realization of its promise for a special tribute only to this field of use, here is a summary of Zeolite exploitation:

  • As soil improvement
  • As ancillary material to increase fertilizers efficiency and fertilization in general
  • As a factor in increasing crop yield
  • As a factor increasing the quality characteristics of crop products
  • As a means of reducing the amount of water supplied to irrigate the crops

Livestock: Zeolite is used as a feed additive for poultry, pigs and cattle. Still, the application of Zeolites to the stables and livestock units in quantities of 2 to 3 kilos per square meter reduces the stench and absorbs ammonia, improving both the living conditions of livestock in the breeding grounds and, on the other hand, making life more enjoyable to the workers. Then, this material can also be used as fertilizer. Even Zeolite is used as deodorant material by adding to the pets sand, reducing its moisture and odor.

Aquaculture: Zeolite is used as a feed additive for fish feed as well as for water purification. Removes ammonia from fish-farming tanks, transport containers, and aquariums while blocking heavy metals and increasing the amount of oxygen in the aerated waters. 

Zeolite in Health

Zeolite is being used both in modern medicine and in traditional healing systems, such as Chinese. Zeolite formulations are used to treat stomach ulcer, but also as regulating agents to reduce the acidity of its content.

The hemostatic effect of Zeolite has been clinically proven, therefore it is a component of similar drugs and is also used as an antioxidant. It is also involved in the synthesis of antidiarrheal and anticholesterolemic formulations, while in the form of powder, it is given for the shorter healing of wounds.

In addition, Zeolite is also used in the manufacture of medical materials such as filters for anesthesia machines – and more.

Chabazite 9mm Pink Rhomb Crystals with Colorless Heulandite
Chabazite 9mm Pink Rhomb Crystals with Colorless Heulandite

Toxicity of Zeolite

If the dose makes the medicine, then the medium makes the use – the right medium. A natural Zeolite has been associated with serious respiratory diseases. Epidemiological studies in villages of Cappadocia in Turkey, whose inhabitants showed high rates of malignant mesothelioma, an uncommon, but an increasingly important neoplasm, showed a correlation of the disease with the natural fibrous form of Zeolite Erionite. The zeolithic volcanic tuffs with Erionite is present in large quantities in this area, and samples of air that were collected contained mineral fibers. Still, an epidemiological study in the same area excluded the correlation of malignant mesothelioma with genetic factors.

References

The References of the entry Zeolite presents the metallic sound production group Metallica with their song Nothing Else Matters. (Nothing Else Matters?)

  1. Auerbach, S. M., Carrado, K. A., & Dutta, P. K. (2003). Handbook of Zeolite Science and Technology. Boca Raton, FL: CRC Press.
  2. Cejka, J., & Bekkum, H. V. (2005). Zeolites and Ordered Mesoporous Materials: Progress and Prospects: The 1st FEZA School on Zeolites, Prague, Czech Republic, August 20-21, 2005. Gulf Professional
  3. DOLL, R. (1989). Mineral fibres in the non-occupational environment: Concluding remarks, in: Non-Occupational Exposure to Mineral Fibres. Lyon: J. Bignon. J. Peto. and R. Saracci (eds.) IARC Science.
  4. Dyer, A. (1984). Uses of natural zeolites. Chemistry and Industry, (7), 241- 245.
  5. Metintas M, Hillerdal G, Metintas S, Dumortier P.(2010). Endemic malignant mesothelioma: exposure to erionite is more important than genetic factors. Arch Environ Occup Health, 65(2), 86-93.
  6. Παπούλης, Δ., Λαμπροπούλου, Π. (2016).Ορυκτολογία: Συστηματική Ταξινόμηση των Ορυκτών [Mineralogy: Systematic Classification of Minerals]. Θεσσαλονίκη: Δίστιγμα.
  7. Στράβων: Άπαντα (Πέμπτος Τόμος). (1994). Γεωγραφικών Ε’: Άλπεις, Ιταλία [Strabo, Geography 5th Book]Αθήνα: Κάκτος.
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