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Problems Identifying Diabase
The fine-grained texture of diabase makes identification in the field or in a classroom a difficult undertaking. The observer must have enough skill to view the physical properties of tiny mineral grains, and enough knowledge to identify the minerals. Getting a confident identification in these locations can be difficult. Diabase is best identified in thin…
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Diabase Porphyry
Some igneous rocks have a complex cooling history that results in them containing grains of distinctly different crystal sizes. These rocks, with large crystals (known as “phenocrysts”) suspended in a matrix of fine crystals (known as “groundmass”), often have a cooling history similar to the following description: The parent magma of the rock began cooling…
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Mineral Composition of Diabase
Diabase typically has a mineral composition that is dominated by the plagioclase feldspar known as labradorite (approximately 40% to 70% of the rock). Most of the remainder is made up of pyroxene minerals (usually augite). Minor amounts of hornblende, olivine, magnetite, and quartz can be found in some diabase rocks. Although the mineral species present in diabase often have perfect cleavage, when they are present in tiny interlocking grains,…
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Diabase
Stonehenge is one of the most famous stone structures ever made by humans. Stones of the monument’s inner circle are made of diabase. They were transported about 240 miles from Wales to the monument site in England in about 2100 BC. What is Diabase? In the United States and Canada, the name “diabase” is used for…
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Blackboards and Chalk
Small pieces of chalk have been used by students for over 1000 years for writing on small slates and large classroom panels known as “blackboards”. It is an inexpensive and erasable writing material and the most widely known use of chalk. Much of the early blackboard writing was done with pieces of natural chalk or…
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Porosity and Permeability of Chalk
At a microscopic level, there can be a lot of space between the fossil particles that make up chalk. Land underlain by chalk directly below the soil is often well drained. In these areas, water that infiltrates into the soil encounters the top of the chalk and easily flows into the chalk’s pore spaces. It…
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Identifying Chalk
The keys to identifying chalk are its hardness, its fossil content, and its acid reaction. At a glance, diatomite and gypsum rock have a similar appearance. An examination with a hand lens will often reveal the fossil content, separating it from gypsum. Its reaction with dilute (5%) hydrochloric acid will separate it from both gypsum and diatomite. The acid reaction will surprise…
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Cretaceous: A Time of Chalk
Much chalk was deposited during the Cretaceous Period of geologic time. It was a time of global high sea levels that began at the end of the Jurassic Period about 145 million years ago and the beginning of the Paleogene Period about 66 million years ago. During the Cretaceous, warm waters of epeiric seas, seas that…
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How Does Chalk Form?
Chalk forms from a fine-grained marine sediment known as ooze. When foraminifera, marine algae, or other organisms living on the bottom or in the waters above die, their remains sink to the bottom and accumulate as ooze. If most of the accumulating organic debris consists of calcium carbonate, then chalk will be the type of rock that…
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What Is Chalk?
Chalk is a variety of limestone composed mainly of calcium carbonate derived from the shells of tiny marine animals known as foraminifera and from the calcareous remains of marine algae known as coccoliths. Chalk is usually white or light gray in color. It is extremely porous, permeable, soft and friable.