Onyx |

Onyx

Abstract

Onyx is a metamorphic rock which is formed from regional metamorphism of limestone that contain Magnesium and Calcium. Onyx is a layered, translucent, cryptocrystalline rock which is available in various colors. In fact Onyx is a subgroup of Marble group in scientific classification. It is mostly used in interior facing and sculpturing. The most important Onyx resources of Iran is located in East Azerbaijan, Khorasan, Kerman and Kurdistan.

Key Words: Metamorphic Onyx, Regional Metamorphism, Marble

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What Is Onyx ?

First of all it should be noted that according to ASTM C119 standard, Onyx marble is a subcategory of Marble group, so to clarify 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 onyx, we mean onyx marble(type 3)

Petrology of Onyx

Onyx is a metamorphic rock which is formed from regional metamorphism of dolomitic limestone that contains clay. In lower to medium grades, Dolomite transforms to Onyx, like Calcite, provided that there is no Silica in the environment.

Limestone with mosaic texture and profile of amphibolite is metamorphic. This category of rocks is translucent and light green. The green color is because of the presence of Actinolite. The beauty of Onyx is because of its color, pattern and most importantly its degree of transparency which gives the stone a rainbow effect. Some Onyx rocks are available with colorful lodes, stripes or waves, but the Onyx that is used for sculpturing is white and relatively pure.

Cipolin: a light-colored Onyx containing layers of micaceous minerals and abundant silicates, some geologists use this word to call all metamorphic limestone.

Mylonite: is a metamorphic rock formed by ductile deformation during intense shearing encountered during folding and faulting.

Onyx stone is always used in sculpturing, building and decorative uses because of its special beauty. Famous artist like Michelangelo and Leonardo da Vinci used onyx stones in their artwork.

Mineralogy

Onyx is formed from metamorphism of limestone or Dolomite. What makes this rock differ from sedimentary carbonate rocks, is its larger crystals. Main mineral component is Calcite, which is often accompanied by Quartz, Graphite, Hematite, Limonite, Pyrite and so forth.

Marbles as a group, are available in a wide variety of colors, but completely pure marble is white and shiny. Range of light grey color to black color is caused by carbonaceous materials. Chlorite and other silicate minerals produce green color in Marble. Pink and red colors are caused by particles of Hematite and Magnesium carbonate, while Limonite produce yellow and creamy colors. Another important factor that influences the color of marble, is environmental conditions when it is forming, which produces white, green, red, pink, yellow and multicolored rocks, like Onyx of Ghezel Ghaya (Kurdistan province), onyx of Takht-e–Kamand-e-Jarquyeh (Isfahan province), Onyx of Chah Zard (Sistan and Baluchestan province), chocolate color Onyx of Ghare Gheshlagh (West Azerbaijan Province).

Physical Analysis

Onyx containing Quartz or other silicate minerals show better strength.
Usually less than 1% water absorption
Average density of 2.7 gr/Cm³
Compressive strength is in range of 19 to 140 MPa nad its average compressive strength is considered 110 MPa.
Flexural strength is in range of 6 to 15 MPa
Average Tensile strength is 4 MPa
Porosity is relatively low and varies from 0.3% to 1.2%
Pure Onyx is white and shiny, but presence of Carbon, Chlorite, Hematite and Limonite, changes the color of Onyx and turns it into black, green, pink, yellow and red.

Advantages and Disadvantages of Onyx

Onyx is very vulnerable to weathering, gases and acidic rain, so Onyx stones are used in interior facings.
Low porosity
Its beauty and wide variety of colors, especially in Iran
Capability of being polished(during production process)
Appropriate shininess
Appropriate for interior facings, sculpturing and artworks.
Nowadays, using bookmatched Travertine and Onyx in floor of lobbies and important sections of buildings are very popular and also beautiful. Patterns of Travertine can create great beauty, provided that Travertine is cut and used in specific direction.
Onyx is vulnerable to air pollution, sulfur gases and humidity, so it is not appropriate for exterior uses in cities with polluted air.

Distribution of Onyx in Iran

Onyx resources are mainly located in Sanandaj-Sirjan zone, and they are more concentrated in Ghorveh, Neyriz, Aligudarz, Ezna, South of Kerman province (Faryab-Arzuiyeh) and northwest, southeast and central areas of this zone.

Different Kinds of Onyx