Dolomite

Dolomite introduction

1. Presentation on Dolomite Presented by: Hammad Ahmad sheikh BS Hons(4 th Prof)

2. Contents Carbonate rocks Minerals Of Carbonate rocks What is dolomite? Physical properties of dolomite Crystal structure Formation of Dolomite Fabric of Dolomite Geochemistry of Dolomite

3. Contents Identification of Dolomite Zoned and unzoned Dolomite Dedolomitization Uses of dolomite Conclusion Reference

4. Carbonate Rocks Carbonates are sedimentary rocks formed at (or near) the Earth's surface by precipitation from solution at surface temperatures. The most common carbonate sedimentary rocks are limestone and dolostone, but also Sodium and Potassium Carbonates can be found. Carbonate rocks are Depositionally most simple but diagenetically most complex rocks of the world.

5. Carbonate Minerals There are a number of a carbonate minerals which are formed by combining of one or more ion with the CO 3 2- ion. Some important minerals are given in the table (1.1) Mineral name Chemical Formula Calcite Aragonite Dolomite Ankerite Magnesite Siderite CaCO 3 CaCO 3 CaMg(CO 3 ) 2 CaFe(CO 3 ) 2 MgCO 3 FeCO 3 Table 1.1

6. Dolomite Dolomite is an carbonate mineral composed of calcium magnesium carbonate CaMg(Co 3 -2 ). Dolostone is a termed use for a sedimentary rock which is formed from such a mineral. Physical Properties of Dolomite Color Hardness Streak Crystal system Luster Tenacity White,grey to pink,yellow 3.5 to 4 White Trigonal Vitreous to Pearly brittle Table No 1.2

7. Crystal Structure It is a rhombohedra carbonate consisting of alternating layers of carbonate anions and the cations. In the crystal structures there is a Ca and Mg ions arranged with Co 3 -2 . Fig 1 showing the crystal lattice of dolomite.

8. Formation o f Dolomite Dolomite is formed by the replacement of the calcite ions by the magnesium ions. Depending upon the ratio of the mg ions in the crystal lattice they have different names. Fig 2 showing the stages of Dolomite formation.

9. Formation of Dolomite Fig 3 showing the formation of high ordered dolomite from high-mg calcite.

10. Formation of Dolomite In the high-Mg calcite there is a 0-32 mol % of Mg substitution for Ca. In the Protodolomite there is roughly 55-60 Mol % Cain the lattice with incomplete segregation of Ca and Mg into separate layers In the stoichiometric dolomite has a 50:50 of Ca to Mg Ratio with near perfect ordering of the Mg ad Ca in alternate cation layers.

11. Formation of Dolomite Anciently Primary dolomite is very rare only forming in certain lakes and lagoons while most of the dolomite is from the replacement origin. Two major considerations in the formation of dolomite are: The source of the Mg+ ions. The process whereby the dolomitizing fluid is pumped through the carbonate sediments.

12. Formation of Dolomite To study the ancient dolomite five broad categories of dolomitization models are currently available which are given below: Evaporative Dolomitization Seepage-reflux Dolomitization Mixing-Zone Dolomitization Burial Dolomitization Seawater Dolomitization Each involves different type of dolomitizing fluid, mode of flow and geological settings.

13. Formation of Dolomite Evaporative Dolomitization : Dolomite is being precipitated within high intertidal supratidal and sabkhas enviornment . Dolomitic that formed in supratidal enviornment are precipitated from evaporated sea water. Early formation of aragonite and gypsum leading to a higher Mg/ Ca ratio of porewater to facilitate the dolomitization .

14. Formation of Dolomite Seepage-reflux Dolomitization : This involves the generation of dolomitizing fluids through evaporation of lagoon water or tidal flat pore water and then descent of these fluids into underlying carbonate rocks. Sea level Saline lagoons and sabkhas Dolomitization Mg bearing fluid

15. Formation of dolomite Mixing Zone dolomitization : This type of dolomite formed by the mixing of sea water with the fresh water. Sea level Reef Materoic vadose zone Meteroic phreatic zone Rain Mixing zone

16. Formation of Dolomite Burial Dolomitization : The principal mechanism is the compactional dewatering of basinal mudrocks and the expulsion of Mg rich fluids into adjacent shelf edge. The transformation of clay minerals with increased burial and rising temperature suggest that it would liberate Mg ions along with Fe ion. Basinal shales are commonly organic rich and the diagenesis would contribute to Co3 ions.

17. Formation of Dolomite Seawater Dolomitization : itself can also be a source of Dolomite because it contains the sufficient amount of Mg ions with little modification if there is an efficient mechanism for pumping it.

18. Fabric of dolomite Nonplaner: Fabrics with irregular, nonlinear, crystal boundaries between anhedral crystals are termed “ nonplanar ”.Based on the nature of crystal boundaries there mosaic is termed as Xenotopic. Planer: Fabric showing planer crystal boundaries with subhedral or euhedral crystal outlines are termed “Planer-e (Idiotopic)” and “Planer-s (Hypidiotopic ) “respectively.

19. Fabric of Dolomite The difference between Planer-e and Planer-s fabric is of shape of the crystals. In planer-e the shape of the crystals are euhedrel . In planer-s the shape of he crystals are sub- hedrel

20. Trace Elements Geochemistry of Dolomite Minor and trace constituents are found in natural calcite,dolomite and aragonite. Trace element in dolomite can be present in a similar manner to the major cations Ca and Mg. The trace elements usually studied are Sr,Na,Fe and Mn . Because of trace elements zoning occurs in dolomite.

21. Trace Elements Geochemistry of Dolomite There are two types of zoning in dolomite: Zoned Dolomite Unzoned Dolomite Zoned dolomite shows the layering of the different minerals in the same crystal that is of generally Zn and Mn . Fig showing the zonation in dolomite crystal

22. Identification of Dolomite In the lab it is best identify by staining it with the Alizarin Red Sulphate Solution. Result . — Color of calcite will turn into red-brown, whereas dolomite will not be affected . In the field it gives effervesence with 10% HCL in the powdered form.

23. Dedolomitization Dedolomitization refers to th e partial to whole transformation of former dolomite rocks to a calcian rich rock. The dolomitization can be considered as a kind of process in which Mg is gradually released. The dedolomitization process can be divided in to two steps Dissolution of Dolomite The precipation of Calcite.

24. Uses of Dolomite Dolomite serves as an oil and gas resorvior rock. Dolomite is used in the construction industry. Dolomite is used in a steel industry. Dolomite is used as a source of Magnesia. Dolomite serves as the host rock for many lead,zinc and copper deposits. Dolomite used as a pigments,paints . Dolomite is used in making of bricks.

25. Conclusions Dolomite is a major carbonate mineral. Its formation from secondary origin is common. Dolomite is generally formed from the replacement of the Ca ions with the Mg ions. In the ancient dolomite the type of its formation can be of different mechanisms it could not be easily understood. Dolomite can behave as the resorvoir rocks. Dolomites have wide uses.