"Chalk it Up" to the Mojave EarthCache
"Chalk it Up" to the Mojave
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Chalk (visit link) is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite (visit link) or calcium carbonate (CaCO3). It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores.
Chalk in the Mojave region [see (visit link) for geological history of the area] formed starting in the late Proterozoic and lasting through all of Paleozoic time (570 to 245 million years ago). The great carbonate-rich (limestone and dolomite) sedimentary rock sections (as much as 10 kilometers thick) preserved in the Mojave region is a testament that during this vast period of time, North America had gradually drifted northward across the equatorial region, home to warm, shallow, continental platform seas teaming with shelly organisms (mostly algae and invertebrates). The chalk was ooze accumulating at the bottom of the great sea. Protozoans such as foraminifera lived on the marine debris that showered down from the upper layers of the ocean. Their shells were made of calcite extracted from the rich sea-water. As they died, a deep layer gradually built up and, eventually, through the weight of overlying sediments, became consolidated into rock. Later earth movements raised these sea-floor deposits above sea level.
Chalk has greater resistance to weathering and slumping than the clays with which it is usually associated, thus forming tall steep cliffs where chalk ridges meet the sea. Chalk hills, known as chalk downland, usually form where bands of chalk reach the surface at an angle, so forming a scarp slope. Because chalk is porous, it can hold a large volume of ground water, providing a natural reservoir that releases water slowly through dry seasons.
Chalk is composed mostly of calcium carbonate with minor amounts of silt and clay. It is normally formed under water, commonly on the sea bed, then consolidated and compressed during diagenesis into the form we commonly see today. During diagenesis, silica accumulates to form chert or flint nodules within the carbonate rock.
It is one of the most common minerals on the face of the Earth, comprising about 4% by weight of the Earth's crust and is formed in many different geological environments. Calcite can form rocks of considerable mass and constitutes a significant part of all three major rock classification types. It forms oolitic, fossiliferous and massive limestones in sedimentary environments and even serves as the cements for many sandstones and shales. Limestone becomes marble from the heat and pressure of metamorphic events. Calcite is even a major component in the igneous rock called carbonatite and forms the major portion of many hydrothermal veins. Some of these rock types are composed of better than 99% calcite.
With calcite so abundant and so widely distributed it is no wonder that it can be so varied. The crystals of calcite can form literally a thousand different shapes by combining the basic forms of the positive rhombohedron, negative rhombohedron, steeply, moderately and slightly inclined rhombohedrons, various scalahedrons, prism and pinacoid to name a few of the more common forms. There are more than 300 crystal forms identified in calcite and these forms can combine to produce the thousand different crystal variations.
The best property of calcite is the acid test. Why? Because calcite always will effervesce (bubble) when even cold weak acids are placed on specimens. Even the cement in sandstones will effervesce assuring the geologist of identification of the cementing mineral. The reason for the bubbling is in the formula below:
CaCO3 + 2H(+1) -------> Ca(+2) + H2O + CO2 (a gas)
The carbon dioxide gas (CO2) is given off as bubbles and the calcium dissolves in the residual water. Any acid, just about, can produce these results, but dilute hydrochloric acid or vinegar are the two recommended acids for this test. Other carbonates such as dolomite or siderite do not react as easily with these acids as does calcite and this leads to differentiating these somewhat similar minerals more readily.
Calcite is intricately tied to carbon dioxide in another way. Since many sea organisms such as corals, algae and diatoms make their shells out of calcite, they pull carbon dioxide from the sea water to accomplish this in a near reverse of the reaction above. This is fortuitous for us, as carbon dioxide has been found to be a green house gas and contributes to the so called "green house gas effect". Environmentally then, calcite is very important and may have been quite important to the successful development of our planet in the past. By pulling carbon dioxide out of the sea water, this biological activity allows more of the carbon dioxide in the air to dissolve in the sea water and thus acts as a carbon dioxide filter for he planet. Environmentalists are now actively engaged in determining if this activity can be increase by human intervention to the point of warding off the "green house gas effect". A significant amount of calcite precipitation in sea water is undoubtedly inorganic, but the exact amount that this contributes is not well known. Calcite and other carbonate minerals are very important minerals in the ocean ecosystems of the world.
To claim this earthcache, you must answer the following questions:
1. Based on the Mohs system of hardness (visit link) , what is the hardness rating of calcite? What are 2 common minerals that calcite could scratch?
2. Add a small amount of vinegar (or other acidic compound) to the calcite at GZ and describe what you see. What is the chemical responsible for what you see?
3. What is the colour and approximate size of the calcite at GZ?
4. What major rock-type is calcite -- sedimentary, igneous, or metamorphic -- and why?
5. Given your answer to Question #4, what is calcite's major derivation or type -- clastic, chemical, or organic -- and why?
Do NOT post your answers, encrypted or otherwise, on your log.
Do NOT log as a find until you have forwarded the answers for this EC. Failure to answer required questions through e-mail will result in a log deletion without notice.
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