Tracing the Origin of Chalk Rocks using X-ray Diffraction
Fingerprinting of sedimentary rock can provide a snapshot to past environments that is essen-tial for studies of the climate and the evolution of life during a set period of time. Much like oth-er limestone rock formations, chalk is a calcite-rich sedimentary rock associated with prehistoric marine environments. Geologically-linked trace minerals within the rock matrices provide unique differences and distinct fingerprints that can be observed via a suitable analytical technique, such as X-ray diffraction (XRD).
Introduction
Chalk rock, more commonly referred to as chalk, is a sedimentary rock mainly composed of calcium carbonate (CaCO3) in the mineral form calcite. Its origin can be traced back to the late Cretaceous era, mainly owing its existence to marine micro-organisms coccolithophores (1). The calcium carbonate-rich remains of these single-celled organisms accumulated on the ocean floor over millions of years and eventually formed thick layers of chalk rock. Although these organisms form the major component of chalk, it also contains some minor impurities in the form of clay minerals, silica, and organic matter that are all bound together by a calcite matrix. The matrix originates from the dissolution and recrystallization of the abundant calcium carbonate, forming a cohesive rock. As a result of the slow lithification process, valuable insights into the geological history of a specific region or location can be obtained by looking at the unique composition of this biogenic sedimentary rock.
In addition, chalk is easily distinguishable by its fine-grained texture, white to light grey color (similar to gypsum), and high porosity. These characteristics make it susceptible to scratching and crumbling which is a distinct difference to other types of limestone.
Given the versatility gained from the properties and abundance of chalk, it has a number of modern-day uses. In research, XRD can be employed to identify, quantify, and ultimately fingerprint the crystallographic composition of chalk specimens linked to a specific location. The fingerprint contains the records of past marine environments that were governed by the changes in the climate and evolution of marine life.
In this application report, chalk samples from different locations were measured using the Automated Multipurpose Powder X-ray Diffractometer, XRDynamic 500, to provide high resolution crystallographic data of the specimens. By analyzing the diffraction data, geological fingerprints for each specimen could be determined and compared with one another.
References
1. Chalk, Geologyscience https://geologyscience.com/rocks/sedimentary-rocks/chalk/, Access on 20.12.2023
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