Gypsum crusts

A priori considerations. An extensive review of the form and mineralogy of the secondary cave deposits is provided by Hill and Forti (1997), but also see Ford and Williams (1989). By far the most useful speleothems for geological purposes are the most common forms stalactites, stalagmites and flowstones composed of calcite, and occasionally aragonite. Other deposits that have been used to derive useful information include travertines, gypsum crusts, calcite veins and scalenohedral calcite (dog-tooth spar). 

Atmospheric acidity is responsible for tbe attack in the presence of humidity whereas the SO2 reaction continues to occur in the presence of liquid water as well as water vapours. The calcium sulfate that forms is less preserved when the structures are exposed to direct rainfall. In protected regions SO2 continues its attack almost continuously and the gypsum obtained from this reaction forms crusts on protected surfaces. As a consequence most ancient buildings in the industrialized countries have a black appearance in some areas and a clean aspect in others. The black appearance is due to gypsum crusts, which have incorporated soot in the process of crystallization whereas the areas directly showered with the... 

Sulphate crust Composed of the superposition of sulphates (e.g., barite, gypsum) on rocks not gypsum crusts that are sedimentary deposits... 

Eckardt, F.D., Drake, N., Goudie, A.S., White, K. Viles, H. (2001) The role of playas in pedogenic gypsum crust formation in the Central Namib Desert a theoretical model. Earth Surface Processes and Landforms 26, 1177-1193. 

Watson, A., 1983a. Gypsum crusts. In Goudie, A. S. K. Pye (eds.) Chemical Sediments and Geomorphology Precipitates and Residua in the Near-surface Environment. Academic Press, New York 133-162. 

Figure 4 shows the occurrence of gypsum crust at the Field Museum of Natural History, Chicago Figure 5 shows that the gypsum has penetrated into the intergranular space this space must have first formed due to the dissolution of calcite. 

Field Museum of Natural History, Chicago. The Caryatid on the left has a black gypsum crust the Cayratid on the right lacks such a crust because it is continually washed by the rain. 

Figure 6 illustrates this phenomenon. Note that the angel under the dome has been rendered unrecognizable by the exfoliation of gypsum crusts formed by weathering. The angel in the open, on the other hand, has been much less severely deteriorated, although it is made of the same marble. 

Cave Hill Cemetery, Louisville. Angels showing exfolliation of gypsum crust in areas under the dome and grain by grain dissociation of marble exposed to direct impact of rain. 

GAURI I believe this is true, but even though this building has been up for about 400 years, I have found only recrystallized calcite in the weathered regions of the stone. This is unusual because most limestone and marble buildings in the industrial countries have gypsum crusts. I am not quite sure but the explanation lies perhaps in a combination of factors such as relative solubilities of calcite and gypsum and the pore characteristics of this limestone. 

Some authors have specifically postulated that these crusts are fornied by combination of sulphates from air pollution and Ca from feldspars [30]. In such case, the combination of Ca from feldspars with atmospheric SO2 comhig from industiial sources some kilometres faraway has been proposed to explain the existence of thick gypsum crusts. However, the low Ca content in the rock (<4%), the low reactivity of Ca-rich feldspars, and the absence of similar crusts in buildings closer to the industrial sources cannot explain the presence of thick gypsum crusts [4]. Some other reports have attributed the crusts to the oxidation of rock pyrites [113], although in most cases, the S content of the silicate rock is very low or absent in the rock (<1%), and pyrites are scarce. 

Another technique of delivering chemical substances and that combines chemical and physical processes is reviewed in Doehne and Price (2010) consist in the use of EDTA delivery through a latex poultice that can be peeled off afterwards that can be used for cleaning of soiling indoors but that apparently does not work in the presence of gypsum crusts and might have the risks of leaving residues. 

Calcium is a metallic element, fifth in abundance in the earth s crust, of which if forms more than 3%. It is an essential constituent of leaves, bones, teeth, and shells. Never found in nature uncombined, it occurs abundantly as limestone, gypsum, and fluorite. Apatite is the fluorophosphate or chlorophosphate of calcium. 

Gypsum, one of the more common sedimentary rocks in the earth s crust, occurs in varieties that differ from each other in texture rather than in composition. In most of its varieties gypsum is very soft and can be scratched even with a fingernail. One massive and fine-grained variety of gypsum, known as alabaster, much appreciated for designing delicate decorative objects, has been used particularly for carving and ornamentation. 

Sulfur constitutes about 0.052 wt % of the earth s crust. The forms in which it is ordinarily found include elemental or native sulfur in unconsolidated volcanic rocks, in anhydrite over salt-dome structures, and in bedded anhydrite or gypsum evaporate basin formations combined sulfur in metal sulfide ores and mineral sulfates hydrogen sulfide in natural gas organic sulfur compounds in petroleum and tar sands and a combination of both pyritic and organic sulfur compounds in coal (qv). 

H. F. Keller has described some of the lakes in the valley of Mexico which leave a crust—called tequezquite, conjitello, tepalcate, and espumilla—consisting of sodium carbonate with sodium chloride, a little nitrate, gypsum, sand, and earthy matter. The product is purified by crystallization to furnish soda crystals—called sosa bnita. In the south of Tezcaco, the country people use tequezquite directly without any purification. P. Benard washed out the sodium chloride with a dil. soln. of the same salt at —18° to —20°, for at this temp, sodium carbonate is virtually insoluble in a sat. soln. of sodium chloride. 

Calcium in fifth in abundance in earth s crust and occurs as chalk, limestone, marble (CaC03), anlydrite (CaS04) and gypsum (CaS04 2H20). 

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