GRAINS: Non-skeletal Grains: Non-Carbonate Grains
2003. "GRAINS: Non-skeletal Grains: Non-Carbonate Grains", A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis, Peter A. Scholle, Dana S. Ulmer-Scholle
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A wide variety of non-carbonate grains can be found as constituents of carbonate rocks. In some cases, these grains are isolated and very subordinate particles; in other examples, they can be important rock-forming elements. Most of these minerals can also be found as diagenetic precipitates in carbonate rocks, but in this section only examples of true grains are illustrated (although some were synsedimentary diagenetic materials that effectively acted as sediment grains).
It is beyond the scope of this book to examine these non-carbonate grains in detail, but a few of the more important types — clastic terrigenous grains, glauconite “pellets”, and ferruginous and phosphatic ooids —will be discussed briefly. The criteria for their recognition and the compositional characteristics of these grains are given in the individual figure captions.
The recognition of non-carbonate grains in carbonate rocks is important for the interpretation of depositional environments and for the proper classification of mixed-composition rocks. Most specialized classifications of carbonate rocks simply use adjectives such as “quartzose”, “very quartzose”, “feldspathic” or “glauconitic” to note the presence and relative abundance of non-carbonate grains (the term “silicified” is appropriately used where a significant amount of silica replacement has affected the deposit).
Well-rounded detrital quartz sand grains are scattered throughout this dolomitized carbonate mudstone. The quartz grains are at various stages of extinction, but none show birefringence colors higher than first order. Such excellent rounding typically indicates a precursor sedimentary source or long-term abrasion in a high-energy setting.
Abundant angular to subangular, detrital quartz grains (and subordinate feldspar) in a sideritic carbonate. Such angular grains are more common as terrigenous contributions to carbonate sediments than the very well rounded grains of the previous example. The siderite crystals are clear to brownish and have high relief.
Large, ovoid, pelletal glauconite grains in a glauconitic marl. The light green color in plane-polarized light and speckled, dark green appearance under cross-polarized illumination are characteristic for glauconite. Although “glauconite” basically is an iron- and magnesium-rich illite-type mineral, the term really refers to a family of related minerals (“glaucony” is sometimes used as a generic term for such materials when detailed mineralogical data is lacking). Glauconite grains can have varying degrees of mineral ordering, as well as a complex range of interlayered clay minerals (especially smectite).
Figures & Tables
A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis
This volume expands and improves the AAPG 1978 classic, A Color Illustrated Guide to Carbonate Rock Constituents, Textures, Cements, and Porosities(AAPG Memoir 27). Carbonate petrography can be quite complicated. Changing assemblages of organisms through time, coupled with the randomness of thin-section cuts through complex shell forms, add to the difficulty of identifying skeletal grains. Furthermore, because many primary carbonate grains are composed of unstable minerals (especially aragonite and high-Mg calcite), diagenetic alteration commonly is quite extensive in carbonate rocks. The variability of inorganic and biogenic carbonate mineralogy through time, however, complicates prediction of patterns of diagenetic alteration. This book is designed to help deal with such challenges. It includes a wide variety of examples of commonly encountered skeletal and nonskeletal grains, cements, fabrics, and porosity types. It includes extensive new tables of age distributions, mineralogy, morphologic characteristics, environmental implications and keys to grain identification. It also encompasses a number of noncarbonate grains, that occur as accessory minerals in carbonate rocks or that may provide important biostratigraphic or paleoenvironmental information in carbonate strata. With this guide, students and other workers with little formal petrographic training should be able to examine thin sections or acetate peels under the microscope and interpret the main rock constituents and their depositional and diagenetic history.
- carbonate rocks
- color imagery
- problematic fossils
- sedimentary rocks