Contr. Mineral. and Petrol. 36, 171--174 (1972) 9 by Springer-Verlag 1972 Monohydrocalcite in Speleothems : An Alternative Interpretation Paul L. Broughton Saskatchewan Department of Mineral Resources Subsurface, Geological Laboratory, 201 Dewdney Avenue East, Regina, Saskatchewan, Canada Received March 30, 1972 Abstract. Genesis for a mineral assemblage from Eibengrotte, a small limestone cave near Frankische Sehweiz, Western Germany, is described. The assemblage includes monohydro- calcite, hydromagnesite, nesquehonite, dolomite, aragonite and calcite. A biochemical genesis is proposed as a growth alternative to the Fischbeek and Miiller (1971) evaporation-aerosol mechanism. Introduction Fischbeck and Mfiller (1971) have described a mineral assemblage from a small German limestone cave (Eibengrotte) which includes carbonates that are relativ- ely uncommon within cave environments. The assemblage includes mono- hydrocalcite ; CaCO3" I-I20 ; hydromagnesite, Mg4(OI-I)2(COs)a. tI20 ; nesquehonite, MgCOa-H20 ; dolomite; aragonite and calcite and occurs as non-detrital in situ growths. Fischbeek and Miiller's paper is important because it brings to fight and additional contribution (monohydroealeite) to the growing list of mineral species known to occur naturally in fimestone-dolomite caves. More than 80 have thus far been recorded (Moore, 1970; Broughton, 1971). Evaporation-Aerosol Genesis The cave-mineral assemblage was compared by Fischbeck and Mfiller with the occurrence of similar mineral assemblages in salt lakes, tropical lagoons, and other saline bodies in arid to semi-arid regions. They contend that the formation either of calcite or aragonite is related to an increase of the Mg/Ca ratio in the solution, and cited the experimental work of Lippman (1960). It was their con- tention (p. 90) that" the precipitation of the whole sequence of carbonate minerals within the cave only occurs if small amounts of solution are evaporating at relatively low humidity". The precipitation of carbonates within caves was assumed to be analogous to the formation of surface carbonates (in semi-arid saline enivironments), wherein precipitation is induced by water evaporation. From an analogy for the occurrence of monohydrocalcite formed in the air spray or an aireonditioning plant (Marsehner, 1969), Fischbeck and Mfiller proposed that within Eibengrotte the monohydrocaleite deposited from an aerosol spray. This species was found to be main component of carbonate scales precip- itated at the interface between cold water and air in the chamber of aircondi-