Editing Limestone (section)

===Other characteristics===
[[File:Seven Sisters 3.jpg|thumb|The [[Beachy Head]] cliffs are composed of chalk.]]
Limestone outcrops are recognized in the [[Field work|field]] by their softness (calcite and aragonite both have a Mohs hardness of less than 4, well below common silicate minerals) and because limestone bubbles vigorously when a drop of dilute [[hydrochloric acid]] is dropped on it. Dolomite is also soft but reacts only feebly with dilute hydrochloric acid, and it usually weathers to a characteristic dull yellow-brown color due to the presence of ferrous iron. This is released and oxidized as the dolomite weathers.{{sfn|Blatt|Tracy|1996|p=295}} Impurities (such as [[clay]], sand, organic remains, [[iron oxide]], and other materials) will cause limestones to exhibit different colors, especially with [[Weathering|weathered]] surfaces.

The makeup of a carbonate rock outcrop can be estimated in the field by etching the surface with dilute hydrochloric acid. This etches away the calcite and aragonite, leaving behind any silica or dolomite grains. The latter can be identified by their [[rhombohedral]] shape.{{sfn|Blatt|Tracy|1996|p=295}}

Crystals of calcite, [[quartz]], [[Dolomite (mineral)|dolomite]] or [[barite]] may line small cavities (''[[vugs]]'') in the rock. Vugs are a form of secondary porosity, formed in existing limestone by a change in environment that increases the solubility of calcite.{{sfn|Blatt|Tracy|1996|pp=315-317}}

Dense, massive limestone is sometimes described as "marble". For example, the famous [[Portoro marble|Portoro "marble"]] of Italy is actually a dense black limestone.<ref>{{cite journal |last1=Fratini |first1=Fabio |last2=Pecchioni |first2=Elena |last3=Cantisani |first3=Emma |last4=Antonelli |first4=Fabrizio |last5=Giamello |first5=Marco |last6=Lezzerini |first6=Marco |last7=Canova |first7=Roberta |title=Portoro, the black and gold Italian "marble" |journal=Rendiconti Lincei |date=December 2015 |volume=26 |issue=4 |pages=415–423 |doi=10.1007/s12210-015-0420-7|s2cid=129625906 }}</ref> True [[marble]] is produced by recrystallization of limestone during regional [[metamorphism]] that accompanies the mountain building process ([[orogeny]]). It is distinguished from dense limestone by its coarse crystalline texture and the formation of distinctive minerals from the silica and clay present in the original limestone.{{sfn|Blatt|Tracy|1996|pp=474}}