Calcite color

Carbonate in boilers usually is present as a hard, dense, white to tan or brown calcite scale (CaC03). A tan to brown color usually indicates the presence of iron. Samples of scales and deposits normally fizz when tested with concentrated muriatic acid (hydrochloric acid, HC1) if carbonate is present, although some preliminary heating may be required. 

The composition of the particles is related to that of the source rocks. Quartz sand [composed of silica (silicon dioxide)], which makes up the most common variety of silica sand, is derived from quartz rocks. Pure quartz is usually almost free of impurities and therefore almost colorless (white). The coloration of some silica sand is due to chemical impurities within the structure of the quartz. The common buff, brown, or gray, for example, is caused by small amounts of metallic oxides iron oxide makes the sand buff or brown, whereas manganese dioxide makes it gray. Other minerals that often also occur as sand are calcite, feldspar and obsidian Calcite (composed of calcium carbonate), is generally derived from weathered limestone or broken shells or coral feldspar is an igneous rock of complex composition, and obsidian is a natural glass derived from the lava erupting from volcanoes see Chapter 2. 

The color of calcite can be due to its iron content, which is present as Fe, to Mn " ", to radiation damage centers, which often involve rare earth... 

Although calcium is more metallic in character than magnesium, compounds of the two elements share some similar properties. Calcium carbonate, CaC03, occurs naturally as chalk and limestone. Marble is a dense form of calcium carbonate that can be given a high polish it is often colored by impurities, most commonly iron cations (Fig. 14.26). The two most common forms of pure calcium carbonate are calcite and aragonite. All these carbonates are the fossilized remains of marine life. Calcium carbonate decomposes to calcium oxide, CaO, or quicklime, when heated ... 

Lapis lazuli is a deep blue gemstone that is a complex copper silicate mineral varying widely in composition. It often contains sparkles of iron pyrite or calcite. The best source is probably Afghanistan. A pale blue variety is found in Chile. Some material sold as lapis lazuli is actually artificially colored jasper from Germany. 

Some carbonate minerals are sources of important elements like zinc (smithsonite) and magnesium (magnesite). Other members of this group are used for carving and ornaments because of their striking colors and patterns. These include calcite, rhodochrosite, azurite, and malachite. 

Gypsum sometimes occurs in finely crystalline masses, which is called alabaster. This material resembles marble or fine limestone in texture and color, and has been used for similar purposes. Gypsum, however, is soluble in water, so most objects made of this material do not last if exposed to moisture in any form. Most objects identified as alabaster are actually made of some form of calcite. 

Lapis lazuli is a metamorphic rock composed of three minerals, blue lazu-rite, white calcite, and brassy pyrite. This stone was highly prized by some cultures, both ancient and modern, because of its intense blue color. Most good-quality lapis lazuli has been mined in Afghanistan and southern Russia, although small deposits have been found elsewhere in the world. Common imitations of lapis lazuli are glass, lazulite (a phosphate mineral), and sodalite (a tectosilicate mineral similar to lazurite). 

Hot springs may contain lots of different minerals in solution. Upon reaching the surface, these waters cool off and the compounds held in solution will precipitate around vents or pools. The most common deposits around hot springs are composed of calcite or common opal. Both of these minerals are colorless when pure, but are often colored by other materials in the water. Oxides of iron are the most common pigments in these situations. In shades of brown, red, and yellow, iron oxide may color an entire deposit, or appear episodically, dyeing some layers and not others. 

Fxcept for the difference in color, crystals of rhodochrosiie. MnCOg. and calcite, CaCO . resemble each othet very sto)iiglv fFig. 8 51 Ci ystals of ammomum sulfate, potassium sulfate, KoSOp aie also very similar to one another in... 

Mn(H20)p,] +, is the stable cationic form of manganese. The hydrated ion is pale rose-pink in color. Representative salts are Mn(N03)2 6H20, MnSO/7HoO, and MnCl2 4H20. These salts and the mineral rhodochrosite, MnCOg, are ail rose-pink or rose-red. Crystals of rhodochrosite are isomorphous with calcite (Chap. 8). 

A visible-scale color banding is immediately obvious in polished sections of speleothems stalactites, stalagmites, and flowstone (see Fig. 4). The colors of the bands of calcite vary from pure white to deep browns with all colors in between. In general the banding is coarse with band widths from a fraction of a millimeter to many millimeters. The transport of humic substances that make up the color bands has clearly varied over the growth history of most speleothems. 

The band of buff-colored, translucent pallisade calcite from 11 mm to 32 mm. Zone C, is relatively event-free, with only a few minor dissolution horizons or detrital drapes (not visible with the naked eye) representing short-term local flood events but not longterm glacial advance flooding. The isotopes show parallel oscillations and are somewhat more enriched than the upper zones of the sample. The oxygen isotopic values lie close to those of modern calcite in the cave, suggesting a moderately cool, wet climate similar to that of today. The oscillations in the carbon isotopic values follow the simple pattern of... 

---