434 E. STEIDTMANN ORIGIN OF DOLOMITE 



Skeats describes thin-seetioiis of some Tertiary'^' coi-al reef materials 

 containing as mucli as 15 per cent MgCO^, Imt no dolomite. 



To what extent^ organic secretions of older rocks contain MgO-bearing 

 calcite lias not 1jeen thoroughly studied. Paleozoic limestones and dolo- 

 mites examined ])y the writer ne\er showed any marked difference between 

 the proportions of calcite and dolomite differentiated by stains and the 

 proportions computed from ciiemical analyses. If notable amounts of 

 ^IgCO. had been in isomorphous nnion with calcite, stains ought to have 

 shown a deficiency of dolomite. 



METHODIC OF DIFF HRES'I JATlSCi CALCITE AND DOLOMITE BY STAINS 



The thin-sections and polished slabs of limestones and dolomites, in 

 which the calcite and dolomite grains are to be differentiated by staining, 

 may be etched first in dilnte acid, in order to bring the two minerals in 

 relief. With polislied slabs, the etching process can be watched closely 

 and stopped as soon as relief has become prominent. In some cases etch- 

 ing aloue is snfficieiit to differentiate calcite from dolomite. For example, 

 dolomite rhombohedrons, \\heii embedded in a compact calcite gromid- 

 mass, are brought into conspicuous I'clief. However, when a limestone 

 or dolomite is not nniform in texture and porosity, etching produces relief 

 even when the rock contains only one mineral. 



Thin-sections of limestones oi" dolomites which are to be stained should 

 be made without the co\eJ'-glass. Their preliminary etching can be ac- 

 complished by a momentary innnersion in very dilute TICl, followed by 

 washing in water to sto]) furtlier corrosion. 



Calcite can l)e stained violet by immersion for two minutes in a modi- 

 fied form of the Temberg solution. The stain, if applied as outlined 

 farther on, is a relial)le incficatoi- ol' calcite c\nd is not affected by size of 

 grain, porosity, or other pliysical conditions. If it is desired to stain 

 both dolomite and calcite, the doh)mite can be staiiied first by immersing 

 the sam|)le in a dilute solution of ITCI — 1 part conccnfj'ated HCl to about 

 100 parts of water, to which a few ch-ops of freshly pi"epa]"ed ])otassium 

 ferri cyanide have been added. In every one of several hundi'ed samples 

 treated in this way by the \\rit('r dolomite turned blue, ^hile sedimentary 

 calcite remained unaffected. Some vein calcites also showed a faint re- 

 action. The blue reaction of the dolomite is, of course, due to its FeO 

 content. 



The results were so consistent for the samples studied that it may fairly 

 raise the question. How general is the FeO content of dolomite and the 



K. W. Skeuts: BiiU. .^Ivis. Comp. Zool., vol. 42. 1903. 

 T'. S. Gool. Siiv\r'.v, I'l-of. Paper OO-D, p. 37. 



