Abstrakt
The Gobholo Cave is one of the ten world's largest granite caves (and the longest on the African continent), abound with relatively rich speleothem deposits. For the first time, a set of instrumental techniques (XRD, SEM/EDS, Raman micro-spectroscopy) was used to characterize the speleothems and also waters (ICP-OES) from which these speleothems have been deposited.
Several types of speleothems (and sedimentary fill) were distinguished and studied: "muddy" flowstone, coralloids, black coatings. The results show that the mass of all studied speleothems is amorphous with a variable chemical composition.
Therefore, the speleothems are rather natural substances than mineral species. Four main groups of substances were distinguished, based on their general chemistry: i) Al-rich substances close to gibbsite composition, ii) Al + Si-rich substances close to allophane composition, iii) Si-rich substances close to opal-A composition, and iv) Mn-rich substances (unspecified Mn oxyhydroxides).
Although the samples are nearly amorphous, poorly crystalline gibbsite and birnessite were found to be possibly present. Based on Raman spectroscopy, strongly altered kerogen was detected in some Al + Si-rich samples.
Raman data indicate that the growth of Gobholo speleothems is most probably tightly related to the activity of microorganisms. Based on collected instrumental data, the problematic issue of "mineral" pigotite (a substance commonly referred to from granite caves but only vaguely defined) is critically discussed in the paper.