Dolomitization 1

1. Euhedral Dolomite

Idiotopic mosaic (planar-e euhedral) of fabric replacive non selective dolomite (grey). The dolomite rhombic crystals are evident because unstained by alizarin red whereas the host rock is an ooidal grainstone stained in pink because it consists of non ferroan calcite. Burial ferroan calcite cement occludes the interparticle space (blue staining with K ferricyanide).

Mississippian, South Wales, UK

2. Dolomitization Timing

Detail of previous image with idiotopic mosaic (planar-e euhedral) of fabric replacive non selective dolomite (grey, unstained by alizarin red). The dolomite rhombs appear broken where they had developed at grain contact. Dolomitization seems to have occurred before burial mechanical compaction.

Mississippian, South Wales, UK

3. Zoned Dolomite

Idiotopic mosaic (planar-e euhedral) of fabric replacive non selective dolomite (grey). Burial ferroan calcite cement occludes the interparticle space (blue staining with K ferricyanide) and precipitated following an irregular rim of non ferroan equant calcite of possible meteoric phreatic origin. Dolomite rhombs show zoned subparallel growth line with likely variable composition.

Mississippian, South Wales, UK

4. Euhedral/Subhedral Dolomite

Idiotopic to hypidiotopic mosaic (planar-e to planar-s) of fabric replacive non selective dolomite (grey). The dolomite rhombs are evident because unstained by alizarin red whereas the host rock is an ooidal grainstone stained in pink because it consists of non ferroan calcite.

Mississippian, South Wales, UK

5. Subhedral Dolomite

Hypidiotopic mosaic (planar-s subhedral) of fabric replacive non selective dolomite (grey). The dolomite rhombs are evident because unstained by alizarin red whereas the host rock is an ooidal grainstone stained in pink because of its non ferroan calcite composition.

Mississippian, South Wales, UK

6. Euhedral Dolomite

Skeletal packstone with a bivalve shell affected by micro-boring and coated by a micrite envelope. In the centre, two rhombic crystals of dolomite show evidence of corrosion.

Thin section kindly provided by T. Geel, Vrije Universiteit, Amsterdam

7. Zoned Dolomite

Hypidiotopic mosaic (planar-s subhedral) of fabric replacive dolomite. The dolomite rhombs display zoned crystals with darker subparallel growth lines due to changes in composition of the dolomitizing fluid during crystal growth.

Thin section kindly provided by T. Geel, Vrije Universiteit, Amsterdam

8. Zoned Dolomite

Previous image viewed in crossed polarisers. Hypidiotopic mosaic (planar-s subhedral) of fabric replacive dolomite. The zoned dolomite crystals display darker subparallel areas due to changes in composition of the dolomitizing fluid during crystal growth.

Thin section kindly provided by T. Geel, Vrije Universiteit, Amsterdam

9. Anhedral Dolomite

Xenotopic mosaic (non planar anhedral) of fabric replacive dolomite. The host rock must have been a skeletal packstone/wackestone with echinoderms. The micrite matrix and possible bioclasts were completely replaced. The high Mg calcite echinoderm fragments and the associated syntaxial calcite cement have partly resisted to the replacive dolomitizing process.

Thin section kindly provided by T. Geel, Vrije Universiteit, Amsterdam

10. Anhedral Dolomite

Xenotopic (non planar anhedral) to hypidiotopic (planar-s subhedral) mosaic of fabric replacive dolomite with sparse euhedral rhombs.

Thin section kindly provided by T. Geel, Vrije Universiteit, Amsterdam

11. Anhedral Dolomite

Xenotopic mosaic (non planar anhedral) of fabric replacive dolomite. The host rock must have been a skeletal packstone/wackestone. The micrite matrix and bioclasts have been completely replaced. Echinoderms and the central undetermined grain have partly resisted to the replacive process and contain euhedral dolomite rhombs.

Thin section kindly provided by T. Geel, Vrije Universiteit, Amsterdam

12. Anhedral Dolomite

Xenotopic mosaic (non planar anhedral) of fabric replacive dolomite. The host rock must have been a skeletal packstone/wackestone with echinoderms. Echinoderm (High Mg calcite) in the centre and the associated syntaxial calcite cement were not affected by the replacive dolomitization.

Thin section kindly provided by T. Geel, Vrije Universiteit, Amsterdam

13. Anhedral Dolomite

Xenotopic mosaic (non planar anhedral) of fabric replacive dolomite. The host rock must have been a skeletal packstone/wackestone with echinoderms. The echinoderms (High Mg calcite) and the associated syntaxial calcite cement were not affected by the replacive dolomitization.

Thin section kindly provided by T. Geel, Vrije Universiteit, Amsterdam

14. Euhedral Dolomite

Peloidal skeletal wackestone partly replaced by euhedral (planar e, idiotopic mosaic) dolomite.

Thin section kindly provided by T. Geel, Vrije Universiteit, Amsterdam

15. Euhedral Dolomite

Previous image viewed in crossed polarised light. Peloidal skeletal wackestone partly replaced by euhedral (planar e, idiotopic mosaic) dolomite.

Thin section kindly provided by T. Geel, Vrije Universiteit, Amsterdam

16. Subhedral Dolomite

Peloidal skeletal wackestone partly replaced by subhedral to euhedral (planar s and planar e, hypidiotopic and idiotopic mosaics, respectively) dolomite.

Thin section kindly provided by T. Geel, Vrije Universiteit, Amsterdam

17. Euhedral Dolomite

Oncoidal peloidal wackestone (calcite in pink because of alizarin red staining) possibly of intertidal environment (voids can be interpreted as fenestrae and are infilled by dolomite crystals) with sparse euhedral dolomite rhombs (grey). Dolomite replacement occurred as < 100 microns size crystals on the micrite matrix and as 0.5 mm size crystals within the voids where most likely a coarser crystal calcite spar had been replaced.

18. Euhedral Dolomite

Detail of previous image (thin section stained with alizarin red) with possibly intertidal fenestrae filled by euhedral/subhedral dolomite crystals of 0.5 mm in size and sparse euhedral dolomite rhombs < 100 microns within the micrite matrix.

19. Euhedral Dolomite

As previous image (thin section stained with alizarin red), this is a detail of possibly intertidal fenestrae filled by euhedral/subhedral dolomite crystals replacing calcite blocky spar associated with sparse euhedral dolomite rhombs < 100 microns within the micrite matrix.

20. Euhedral Dolomite

Oncoidal peloidal wackestone (calcite in pink because of alizarin red) with sparse euhedral dolomite rhombs (grey). Dolomite replacement occurred on the micrite matrix as < 100 microns size crystals.

21. Dolomite Cement

Coated grain-ooidal (dolo)grainstone with an isopachous layer of dolomite rhombic crystal (grey) cement lining grains and interparticle pore space. In some cases, the interparticle space is occluded by a hypidiotopic mosaic of dolomite crystals; in other interparticle pore spaces calcite spar (pink) is present.

22. Dolomite Cement

Detail of previous image showing coated grain-ooidal (dolo)grainstone with an isopachous rim of dolomite cement (grey) lining grains and interparticle pore space. Calcite spar (pink) fills the remaining pore space.

23. Dolomite Cement

Detail of previous image showing coated grain-ooidal (dolo)grainstone with an isopachous rim of dolomite cement (grey, alizarin red unstained) lining grains and interparticle pore space. Calcite spar (pink) fills the remaining pore space.

24. Dedolomite

Ooidal grainstone (pink, alizarin red staining) with fabric replacive, dolomite crystals 1-2 mm in size. Dolomite crystals appear affected by corrosion, calcite replacement in pink (dedolomitization) and oxide inclusions because of their instability in a meteoric superficial setting (outcrops).

Mississippian, South Wales, UK

25. Dedolomite

As previous photo, ooidal grainstone (pink, alizarin red staining) with fabric replacive dolomite crystals, 1-2 mm in size. Dolomite crystals appear affected by corrosion and oxide inclusions and show calcitic patches (pink) overlying the dolomitic (grey) crystalline structures.

Mississippian, South Wales, UK

26. Dedolomite

Ooidal (dolo)grainstone (pink, alizarin red staining) with 1-2 mm in size, fabric replacive dolomite crystals. Dolomite crystals are clearly affected by a replacive process of transformation of dolomite into calcite with numerous calcitic patches (pink) overlying the dolomitic (grey) crystalline structures.

Mississippian, South Wales, UK

27. Dedolomite

As previous photo, ooidal (dolo)grainstone (pink, alizarin red staining) with 1-2 mm in size, fabric replacive dolomite crystals. Dolomite crystals are affected by a replacive process of transformation of dolomite into calcite with numerous calcitic patches (pink) overlying the dolomitic (grey) crystalline structures.

Mississippian, South Wales, UK

28. Subhedral Dolomite

Crystals of radiaxial fibrous calcite, 2-3 mm in height, are lining a clotted peloidal microbial boundstone facies (light pink). Subhedral hypidiotopic mosaics of dolomite (grey/light green) have replaced the radiaxial fibrous marine cement. Turquoise/green is a common colour of ferroan (burial) dolomite after staining with K ferricyanide (Dickson, 1965).

Pennsylvanian, Asturias, N Spain, cf. Bahamonde et al. (2015, 2017)

29. Anhedral Dolomite

Skeletal peloidal grainstone to packstone with 50 microns size dolomite crystals (grey/green) forming a xenotopic mosaic over ferroan calcite cement (light blue). Turquoise/green is a common colour of ferroan (burial) dolomite after staining with K ferricyanide (Dickson, 1965).

30. Anhedral Dolomite

Replacive dolomite forming < 50 microns crystals (green/grey) next to two fractures filled by burial ferroan calcite (blue with K ferricyanide staining).

31. Anhedral Dolomite

Xenotopic mosaic of dolomite (grey) replacive probably on the micrite matrix. In the centre a biomould filled by calcite sparite (dark pink).

32. Subhedral Dolomite

Micrite matrix with secondary porosity (blue epoxy) partly replaced by a hypidiotopic mosaic of dolomite (white).

33. Euhedral Dolomite

Idiotopic planar-e mosaic of dolomite rhomb crystals (white) replacing micrite. The blue areas are secondary porosity due to micrite dissolution.

34. Subhedral Dolomite

Hypydiotopic mosaic of dolomite rhomb crystals (white) replacing a skeletal packstone. The blue areas are secondary porosity due to micrite dissolution. A stylolite is filled by bitumen.

35. Subhedral Dolomite

Coated grain (micritised ooids and cortoids) grainstone with sparse euhedral to subhedral dolomite. Dolomite rhombs at grain contacts seem to have been affected by compaction. This might suggest that dolomitization took place before mechanical compaction.

Mississippian, South Wales, UK

36. Subhedral Dolomite

Subhedral to euhedral crystals of dolomite preferentially replacing the micritised grains. The lack of dolomite on the calcite spar cement and the corroded aspect of dolomite at the contact with the calcite spar suggest that dolomite preceded the burial sparite precipitation.

Mississippian, South Wales, UK

37. Euhedral Dolomite

Euhedral dolomite at grain contacts that might have precipitated pre-compaction.

Mississippian, South Wales, UK

38. Subhedral Dolomite

Hypidiotopic mosaic of subhedral dolomite crystals.

Permian, Texas, USA

39. Saddle Dolomite

Saddle (or baroque) dolomite is characterised by curved crystals (due to warped crystal lattice), curved cleavage planes and undulose extinction. It is commonly Ca-rich and may have high Fe content and it forms at high (around 100ºC) temperatures (Tucker and Wright, 1990).

Permian, Texas, USA

40. Saddle Dolomite

Previous image of saddle dolomite viewed in crossed polarised light showing the undulose extinction.

Permian, Texas, USA

41. Saddle Dolomite

Saddle (or baroque) dolomite is characterised by curved crystals (due to warped crystal lattice), curved cleavage planes and undulose extinction. It is commonly Ca-rich and may have high Fe content and it forms at high (around 100ºC) temperatures (Tucker and Wright, 1990).

Permian, Texas, USA

42. Saddle Dolomite

Previous image viewed in crossed polarised light. Saddle dolomite is characterised by curved crystals and undulose extinction.

Permian, Texas, USA

43. Subhedral Dolomite

Hypidiotopic mosaic of replacive euhedral to subhedral dolomite associated with silicification.

Permian, Texas, USA

44. Subhedral Dolomite

Previous image viewed in crossed polarised light. Hypidiotopic mosaic of replacive euhedral to subhedral dolomite associated with silicification.

Permian, Texas, USA

45. Subhedral Dolomite

View in crossed polarised light of a hypidiotopic mosaic of replacive dolomite. The host rock has been completely replaced.

Permian, Texas, USA

46. Subhedral Dolomite

Hypidiotopic mosaic of dolomite.

Permian, Texas, USA

47. Saddle Dolomite

Saddle (baroque) dolomite showing curved cleavage planes.

Permian, Texas, USA

48. Anhedral Dolomicrospar

Peloidal pack-grainstone with meniscus micritic cement binding grains followed by fine (<100 microns) calcite spar (pink with Alizarin red). Dolomicrospar (<50 microns; gray) has formed within some of the micrite grains in an intertidal inner ramp.

Lower Jurassic, High Atlas, Morocco, cf. Merino-Tome' et al. (2012)

49. Anhedral Dolomicrospar

Peloidal grainstone with binding micritic crusts deposited in an intertidal inner ramp. Sparse dolomicrospar occurs locally on the micritic grains.

Lower Jurassic, High Atlas, Morocco, cf. Merino-Tome' et al. (2012)

50. Anhedral Dolomite

Peloidal grainstone with isopachous rims of marine cement (pink with alizarin red) and sparse xenotopic mosaic of dolomite (grey) replacive on the micrite grains.

Lower Jurassic, High Atlas, Morocco, cf. Merino-Tome' et al. (2012)

51. Mimetic Dolomicrite

Lower Jurassic intertidal inner ramp fenestral peloidal wackestone with mimetic dolomicrite. Dolomicrite might be of early syn-sedimentary origin in the intertidal setting. Voids (fenestrae) are infilled by calcite spar.

Lower Jurassic, High Atlas, Morocco, cf. Merino-Tome' et al. (2012)

52. Mimetic Dolomicrite

Lower Jurassic intertidal inner ramp fenestral peloidal wackestone with mimetic dolomicrite (grey). Dolomicrite might be of early syn-sedimentary origin in the intertidal evaporative setting. Fenestrae are geopetally filled by a dolomite silt and calcite spar (pink).

Lower Jurassic, High Atlas, Morocco, cf. Merino-Tome' et al. (2012)

53. Mimetic Dolomicrite

Non obliterative dolomicrite (grey, thin section stained with both alizarin red and K ferricyanide) penecontemporaneous to the deposition of the peloidal packstone. Voids are filled by ferroan (burial) calcite spar (blue).

Lower Jurassic, High Atlas, Morocco, cf. Merino-Tome' et al. (2012)

54. Mimetic Dolomite

Non obliterative dolomicrite (grey, thin section stained with both alizarin red and K ferricyanide) penecontemporaneous to the deposition of the ooidal grainstone. Interparticle space includes euhedral dolomite rhombs (white).

Lower Jurassic, High Atlas, Morocco, cf. Merino-Tome' et al. (2012)

55. Mimetic Dolomite

Non obliterative dolomicrite (grey, thin section stained with both alizarin red and K ferricyanide) penecontemporaneous to the deposition of the peloidal packstone. Voids are filled by calcite spar (pink).

Lower Jurassic, High Atlas, Morocco, cf. Merino-Tome' et al. (2012)

56. Mimetic Dolomite

Pisoidal grainstone from the supratidal zone of the inner ramp with mimetic dolomicrite (grey, thin section stained with both alizarin red and K ferricyanide). Calcite spar cement has a slight blue tone and might be of burial origin (ferroan calcite).

Lower Jurassic, High Atlas, Morocco, cf. Merino-Tome' et al. (2012)

57. Mimetic Dolomite

Non obliterative dolomicrite (grey, thin section stained with both alizarin red and K ferricyanide) penecontemporaneous to the deposition of the fenestral peloidal packstone in the intertidal zone. Fenestrae are occluded by calcite spar (pink).

Lower Jurassic, High Atlas, Morocco, cf. Merino-Tome' et al. (2012)

58. Mimetic Dolomite

Non obliterative dolomicrite (grey, thin section stained with both Alizarin red and K ferricyanide) on a skeletal peloidal packstone. Interparticle space is occluded by calcite spar (pink).

Lower Jurassic, High Atlas, Morocco, cf. Merino-Tome' et al. (2012)

59. Dedolomitization

Stained thin section (alizarin red) showing an euhedral crystal of dolomite (rhombic shape and unstained) replacing an ooidal grainstone. The dolomite is affected by dedolomitization due to exposure to meteoric conditions (telogenesis) as evidenced by the calcitic (pink) irregular patches forming on the dolomite crystal. Field of view approximately 1.5 mm wide.

Mississippian (Chadian), South Wales, UK

60. Dolomite cement

Skeletal grainstone with bioclast moulds surrounded by micrite envelope. The thin section is stained with alizarin red and it is possible to distinguish the calcite grains and cement from the rhombic dolomite crystal. This dolomite is late diagenetic cement in the intergranular pore space (blue epoxy). Minor mouldic porosity is also present.

Upper Cretaceous (Turonian), Miskar Field, offshore Tunisia

61. Mimetic dolomite

This dolo-packstone to grainstone consists of peloids that have been subjected to early micro-crystalline dolomitization that has not obliterated the fabric (mimetic). Limpid dolomite cement lines the fenestral pore, surrounded by mimetic dolomite. The calcite spar occludes the pore space following dolomite cement precipitation. Field of view approximately 1.6 mm wide.

Late Precambrian, Oman

62. Etched dolomite

On the right, the ooidal grainstone was partly replaced by euhedral dolomite. The dolomite seals the grain contacts and must have formed after compaction. Dolomite crystals appear to have been corroded before the precipitation of calcite poikilotopic cement. On the left, image in cathodoluminescence shows carbonate grains (ooids and grapestones) micritized and luminescent, dolomite crystals display bright luminescence in the corroded edges and the late diagenetic poikilotopic spar is not luminescent. Field of view approximately 1.5 mm wide.

Mississippian, Gully Oolite, South Wales, UK

63. Etched dolomite

Euhedral dolomite crystals replacing a coated grain grainstone (pink with alizarin red staining). Dolomite crystals show etched edges in the portion grown in the former interparticle pore spaces where poikilotopic cement precipitated following dolomitization.

Mississippian, South Wales, UK

64. Etched dolomite

Dolomite crystals replacing coated grain grainstone (pink with alizarin red staining). Dolomite crystals show etched edges in the portion grown in the former interparticle pore spaces where poikilotopic cement precipitated following dolomitization.

Mississippian, South Wales, UK

65. Dolomization phases

Euhedral dolomite (idiotopic mosaic planar-e) overprinting anhedral (xenotopic) dolomite along a microstylolite.

Eocene, Mukta Field, offshore India

66. Euhedral dolomite

Mostly euhedral dolomite crystals forming an idiotopic planar-e mosaic. To notice the intercrystalline porosity evidenced by the blue epoxy.

Upper Cretaceous (Turonian), Miskar Field, offshore Tunisia

67. Euhedral dolomite

Mostly euhedral dolomite crystals forming an idiotopic planar-e mosaic. To notice the intercrystalline porosity evidenced by the blue epoxy.

Upper Cretaceous (Turonian), Miskar Field, offshore Tunisia

68. Euhedral dolomite

Idiotopic planar-e mosaic of dolomite leaving open intercrystalline porosity. Field of view approximately 1.6 mm wide.

Carboniferous, Karachaganak Field, Kazakhstan

69. Mimetic dolomite

Mimetic dolomite replacing a peloidal fenestral grainstone and dolomite cement lining pore space. Field of view approximately 1.6 mm wide.

Zechstein Limestone, Permian, UK

70. Dolomite and anhydrite

Anhydrite and euhedral dolomite formed during burial diagenesis viewed in plane polarized light. Field of view approximately 1.6 mm wide.

Upper Cretaceous (Turonian), Miskar Field, offshore Tunisia

71. Dolomite and anhydrite

Anhydrite and dolomite formed during burial diagenesis in plane polarized light. Field of view approximately 1.6 mm wide.

Upper Cretaceous (Turonian), Miskar Field, offshore Tunisia

72. Dolomite and anhydrite

Anhydrite and dolomite formed during burial diagenesis in cross-polarized light. Field of view approximately 1.6 mm wide.

Upper Cretaceous (Turonian), Miskar Field, offshore Tunisia

73. Dolomite and anhydrite

Anhydrite and dolomite formed during burial diagenesis in cross polarized light.

Upper Cretaceous (Turonian), Miskar Field, offshore Tunisia

74. Saddle dolomite

Saddle (baroque) dolomite that formed in burial diagenetic environment viewed in cross polarized light showing undulose extinction. Field of view approximately 1 mm wide.

Jurassic, N Spain

75. Saddle dolomite

Saddle (baroque) dolomite that formed in burial diagenetic environment viewed in plane polarized light showing curved cleavage surfaces. Field of view approximately 1 mm wide.

Jurassic, N Spain

76. Saddle dolomite

Saddle dolomite (unstained) enveloping bladed calcite (stained pink with alizarine red) in a fracture in a skeletal (rotalid foraminifera) packstone.

Eocene, Mukta Field, offshore India

77. Saddle dolomite

Saddle dolomite in a vein in a skeletal packstone. Both have been leached and the clay mineral dickite has filled the late corrosion pore space. Image in cross polarisers show saddle dolomite undulose/sweeping extinction.

Eocene, Mukta Field, offshore India

78. Saddle dolomite

Previous image in plane polarisers. Saddle dolomite in a vein in a skeletal packstone. Both have been leached and the clay mineral dickite has filled the late corrosion pore space.

Eocene, Mukta Field, offshore India

79. Saddle dolomite

Image in cross polarisers of corroded saddle dolomite in a fracture showing undulose/sweeping extinction.

Eocene, Mukta Field, offshore India

80. Saddle dolomite

Saddle dolomite in plane polarizers.

Eocene, Mukta Field, offshore India

81. SEM dolomite

SEM image of dolomite crystals forming an idiotopic to hypidiotopic (dolomite planar-e and –s type) mosaic.

Carboniferous, Karachaganak Field, Kazakhstan