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Metamorphic Rocks
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Some metamorphic rocks are so fine-grained that seeing individual mineral crystals requires a microscope. These rocks may contain no visible layering or fractures and appear as a homogeneous mass. Geologists use the general term hornfels for any dark-colored fine-grained metamorphic rock that does not show visible layering. Most hornfels are quite hard and durable, because constituent grains are tightly bound together. Biotite hornfels, the most common kind of hornfels, are dark brown and sometimes have a slight sheen due to microscopic grains of biotite. Other hornfels may have different colors; the color depends on the rock composition. Some hornfels contain grains that become visible if a rock is weathered (because different minerals weather in different ways) but, because of the generally uniform rock color, are invisible otherwise.
Recognising the different types of building stone
Published in John A. Hudson†, John W. Cosgrove, Understanding Building Stones and Stone Buildings, 2019
John A. Hudson†, John W. Cosgrove
There are two fundamental types of metamorphic rock. The most common forms in response to a combination of heat and tectonic stress; such rocks are said to have been formed by ‘regional metamorphism’. The other type forms in response to an increase in heat alone and typically occurs adjacent to a hot intrusive igneous rock; consequently, such metamorphism is termed ‘contact metamorphism’. The main difference between the two types is that in contact metamorphism the newly formed mineral crystals display a random orientation, because there is no stress acting to cause any alignment. In addition, the crystals are locally developed around the intrusion where they form hard rims. Such rocks are termed hornfels. In contrast, and as the name implies, regionally metamorphosed rocks occur over large areas and often possess a marked mineral fabric.
Crushed rock products
Published in W. A. Peck, J.L. Neilson, R.J. Olds, K.D. Seddon, Engineering Geology of Melbourne, 2018
In the unweathered state, hornfels is a hard, dense, dark grey to black and very fine-grained rock. The bulk particle density approximates 2.7 t/m3. Extreme variations in rock quality may occur due to the fact that interbedded sediments of different type, often steeply dipping, have reacted differently to metamorphism and subsequent weathering. The quartz-biotite hornfelses and the associated quartzites tend to be more resistant to weathering than the cordierite hornfelses derived from argillaceous sediments. The effects of weathering are most intense near the surface but may extend to considerable depths caused by the channelling of groundwater along joints, faults and the more permeable beds. This latter effect may produce a thin weathered skin surrounding the fresh blocks of rock. Variations in quality may also occur due to hydrothermal alteration generated by the nearby intrusive rocks. This type of alteration is evident in the Lysterfield hornfels. In hornfels quarries, the thickness of weathered rock overlying fresh rock may be in excess of 50 metres.
Soil moisture and density monitoring methodology using TDR measurements
Published in International Journal of Pavement Engineering, 2020
Habibullah Bhuyan, Alexander Scheuermann, Didier Bodin, Rolf Becker
The typical UBG road material used in base or sub-base construction in pavement was investigated in this study (Figure 4(a)). The material was factory-made at a quarry in South-East Queensland and it was well-graded gravel (GW) according to technical specification of department of transport and main roads (MRTS05, 2015). The particle size distribution (PSD) has been presented in Figure 4(b). The sample had a fine-grained contact metamorphic source rock of hornfels origin.