Marble |

Marble

Abstract

The word marmarit is exclusive to Iran, it is also called “sanandaji” in commercial literature. In scientific texts Onyx and marmarit are categorized in one group as Marble. Marmarit sometimes includes limestone which is not categorized as Marble. Marble is a metamorphic rock that forms when limestone is subjected to the heat and pressure of metamorphism. It is composed primarily of the mineral Calcite. Marble is very precious because of its beauty, strength and resistance to fire and erosion. Ancient Iranian and Greek people frequently used it in their sculptures and buildings.

In comparison to Travertine, Marble is more appropriate for interior and exterior facings, because of its more hardness and less porosity. Two main textures of Marble rocks include sugary texture and mosaic texture. Pure Marble is white, but presence of impurities like Hematite, Limonite and Carbon turns it into red, light yellow, black, pink and other color combinations.

Key Words: Marmarit, Marble, Dense Limestone

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What is Limestone Marble?

First of all it should be noted that according to ASTM C119 standard, Limestone marble is a subcategory of Marble group, so to rectify any mistaken interpretation, ASTM definition is brought here:

Marble Group: All stone here defined as marble must be capable of taking a polish. Stone in this category comprises a variety of compositional and textural types, ranging from pure carbonate to rocks containing very little carbonate that are classed commercially as marble (for example, serpentine marble). Most marbles possess an interlocking texture and a range of grain size from cryptocrystalline to 5 mm.

Marble: carbonate rock that has acquired a distinctive crystalline texture by recrystallization, most commonly by heat and pressure during metamorphism, and is composed principally of the carbonate minerals calcite and dolomite, singly or in combination.
Limestone Marble: compact, dense limestone that will take a polish is classified as marble in trade practice. Limestone marble may be sold as limestone or as marble.
Onyx Marble: translucent, generally layered, cryptocrystalline calcite with colors in pastel shades, particularly yellow, brown, and green. Onyx marble is formed by slow precipitation from generally cold solutions of carbonated (carbon-dioxide saturated) spring water.

To be specific, in this article by marble, we mean Limestone marble(type 2)

Marble is a metamorphic rock that forms when limestone is subjected to the heat and pressure of metamorphism. It is composed primarily of the mineral calcite and usually contains other minerals such as: clay minerals, micas, quartz, pyrite, iron oxides and graphite. Under the conditions of metamorphism the calcite in the limestone recrystallizes to form a rock that is a mass of interlocking calcite crystals.

ASTM Definition

Marble is a rock with sedimentary origin which is formed from pressure of layers that are over it and weak metamorphism may also be observed in its structure. Marble is a fine-grained to medium-grained rock which has sugary texture.

Petrology of Marble

Marble is an Organogenic sedimentary rock which is categorized as chemical sedimentary rocks. These rocks are mostly formed chemically/biochemically from algae, coral and Calcite sediments. They may also form from Shale, planktonic creatures and silicification. Fossils may be found in layers of this rock, so marbles may be classified as fossiliferous limestone. Generally dimension stones of this group include compact Limestone and fossiliferous limestone.

Mineralogy and Engineering Properties of Marble

Marble is composed primarily of the mineral Calcite and Dolomite and usually contains other minerals such as: clay minerals, Quartz, Hematite, Limonite and Carbon. Marbles are named in respect to the amount of Magnesium/Calcium carbonate they contain, and they are called Calcite Marble or Magnesite (Dolomitic) Marble.

Pure Marble is White and shiny, but Carbon impurities produce light grey to black colors and Chlorite and other silicates produce green color. Red color in Marble originates from Hematite or Magnesium carbonate particles. Finally creamy color is produces by Limonite.

Due to presence of Magnesium, density of Marble is greater than ordinary limestone. Hardness of Calcite is about 3 bud hardness of Magnesium carbonate is about 3.5-4, so Marble rocks that contain Calcium and Magnesium are harder. Marbles consisted of pure Calcite which are used in stairways or facings are easily crushed, while Marble rocks that contain Magnesium carbonate are less vulnerable. Porosity of Marble rocks is about 0.0002% – 0.5%, which is a very low. This means that water absorption is low in Marble stones but makes them resistant to weathering.

There may be Fossils in Marble rocks, which makes it very beautiful when polished, provided that these fossils do not change physical or chemical properties of the stone. In Iran, these rocks are found in different parts of the country.

Dense Limestone in Iran

Pelagic limestone (red), Marble of Gavshan (Sanandaj-Kamyaran road), marble of Najafabad, Marble of Bavanat, Marble of Dehbid (Fars province), Marble of Baba Heydar (Chaharmahal and Bakhtiari Province), Marble of Ghader Abad (Sistan and Baluchestan province), Marble Kahnuk (Sistan and Baluchestan province) and black Marble of Khoy.

Geographical Distribution of Marble Rocks in Iran

Pelagic Limestone are usually found in Ophiolitic zone accompanied by color mélange, this rock is fine-grained and formed from shale and plankton after silicification.

This Kind of rocks in Zagros zone are accompanied by ophiolitic mass and color mélange, especially Pelagic limestone in Kermanshah zone in Kermanshah province date from Eocene and Marble of Divabe Darreh in Kurdistan (Sanandaj-Sirjan zone) date from cretaceous. In West Azerbaijan, Khoy region red Marbles are discovered that date back to Miocene era, and also in Chaharmahal and Bakhtiari Province, most of Marble mines date back to cretaceous era (Bangestan).

Advantages of Marble as Building Stone

1- Decent Hardness and resistance to fire and erosion

2- Suitable for floors, stairways and interior facings, because of its very low porosity

3- Beauty and its color variety especially in Iran

4- Applicable to modern and classic facings

5- Capability of being polished (during production process)

6- Appropriate shininess

7- No pores

Disadvantages of Marble as Building stone

1- It is very expensive, especially the white color

2- Marble is less recommended for exterior facings than Travertine, it is because of its higher density which is in contrast with modern building methods that emphasize on reducing weight of buildings.

3- Presence of cracks reduces durability and strength of the stone and its resistance to freezing

4- It is stained easily, because its pores and the fact that filler cannot be used

5- Exposure to colorful materials changes its texture color, and it is only rectified be re-polishing or changing the stone.

6- Exposure to sunlight, changes its color in long-term

Marble Resources

There are totally 2.165 million tonnes of Marble resources in Iran, the most important provinces that produce Marble include Markazi, Chaharmahal and Bakhtiari, Isfahan, Kurdistan and so forth. It is worth mentioning that the other important producer of Marble in the world is Italy.