For use as stone, carbonate rock should be physically sound, dense, and 

 relatively pure. Porosity of limestone generally ranges from approximately 

 1 to 15 percent. Limestones have an average unit weight of approximately 

 22.0 to 25.9 kilonewtons per cubic meter (140 to 165 pounds per cubic 

 foot). Marble has an average unit weight of 25.1 to 26.7 kilonewtons per 

 cubic meter (160 to 170 pounds per cubic foot). Carbonate stone that is 

 tough, strong, and durable is well suited for use as concrete aggregate, 

 riprap and quarrystone armor units. 



e. Sandstone . Sandstone is clastic sedimentary rock composed of 

 particles mainly in the size range of about 0.25 to 6.4 millimeters (0.01 

 to 0.25 inch) in diameter. Although some sandstones consist almost 

 wholly of quartz grains, most sandstones are feldspathic; some contain a 

 high proportion of ferromagnesian minerals. The strength and durability of 

 sandstone are mainly determined by the type of material that cements the 

 grains together. Only well -indurated sandstone, cemented with silica or 

 calcite (rather than with the weaker cements, clay or iron oxide), is well 

 suited to use as crushed and broken stone. The porosity of sandstone is 

 typically high, ranging from 5 to 25 percent. The average unit weight 

 ranges from approximately 21.2 to 25.1 kilonewtons per cubic meter (135 to 

 160 pounds per cubic feet) . 



f . Miscellaneous Types of Stone . 



(1) Chert . Chert is a sedimentary rock composed almost entirely 

 of silica, in the form of opal, chalcedony, or microgranular quartz. It 

 commonly occurs in thin-bedded deposits. The most desirable form of chert 

 is hard and dense, which is well suited for use as crushed and broken stone 

 and riprap. Some chert, however, is too laminated or contains too much 

 silt or shale for such use. It is generally not used for large quarrystone 

 armor units. 



(2) Conglomerate . This stone is clastic sedimentary rock containing 

 abundant fragments of pebble size or larger in a matrix of sand and finer 

 grained materials. Conglomerates show various degrees of induration which 

 depend largely on the nature and amount of cementing material--clay, calcium 

 carbonate, iron oxides or silica--in the matrix. 



Conglomerate is not abundantly used for riprap or quarrystone armor 

 because relatively few deposits of conglomerate are sufficiently well 

 indurated for this use. 



(3) Greenstone . Greenstone is a general term applied by geologists 

 to basic or intermediate volcanic rocks that contain abundant green secondary 

 minerals. In the stone industry the term is also applied to a variety of 

 fine-grained green rocks, including arkosic sandstone, graywacke, impure 

 quartzite and various pyroclastic rocks. Physically sound greenstone may 



be used for aggregate, riprap, or quarrystone armor, if it is available 

 economically. 



(4) Serpentine . This stone is an ultrabasic igneous rock composed 

 mainly of the mineral serpentine, a hydrous magnesium silicate. Serpentine 

 rock is moderately soft, but commonly massive and dense in structure, and 



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