Role of Microbes in Genesis and Weathering of Sulfur Defjosits 175 



Weathering in sulfur beds and mines of sulfidic ores begins at 

 the outcrop. The sulfur beds of Gaurdagli, District of Chardzliuj, 

 Turkmenian SSR (Fig. 3) are situated in a territory of arid 

 climatic conditions with about 120 mm of precipitation yearly 

 during the summer months. Weathering and oxidation happen 

 mostly during this time of the year. Tlie pH value of tlie water 

 is about one. The limestone shows heavy weathering. Even from 

 great distances, the light color of the weathered limestone is 

 remarkable (Fig. 1). The microbiological weathering is caused 

 by TJiiobact. thiooxidans present at numbers of 10^ to 10^ cells/g 

 limestone. The uppeniiost layers of the sulfur bed of Shor-su 

 point out the same weathering. 



Microbiological weathering is stopped by dryness, bv ab- 

 sence of surface-water, by high levels of groundwater which is 

 poor of oxygen, or by HlS. Karawaiko (5) also found, in the 

 sulfur beds of Rosdol Uki^anian SSR, that surface contact, weath- 

 ering and occurrence of Thiohact. thiooxidans coincide (Table 

 3). 



American scientists (1, 8, 11) have first found the relations 

 between the weathering (oxidation) of Pyrite, Marcasite and 

 other sulfidic ores and the activity of Thiohact. feirooxidans and 



TABLE 3 



Distribution of Thiohact. thiooxidans in Fresh and Weathered Sulfur 

 Containing Limestone (pH of Water About 1). 



Number of 

 Thiobacterium 

 Sample pH of Sample (uer Gm. Limestone) 



(1) weathered sulfur containing limestone 7.64 10^ 



(2) sulfur containing limestone with Alunitc 6.90 10^ 



(3) loamy sediment below the zone of (1) 5.81 10* 



(4) fresh sulfur contraining limestone 7.50 



Thiohact. thiooxidans. In the USSR Ljalikowa (9) has found 

 that Thiohact. fcrrooxidans has its redox-optimuni between rllj 

 22 and 30. This species is spread all over t!ie earth in coal seams 

 with iron sulfides and in mines of sulfidic ores. Tlie oxidation 

 is a combined biochemical and cliemical process witli tlie forma- 



