4-90 Branncr — Diamonds and Ca?'bonados, Brazil. 



and their associated minerals, there is no great geographic 

 difficulty in accounting for them. 



But if they originated in the ancient rocks and passed down 

 with their associates through ages to the Lavras series, the 

 hardness of the minerals must have been a factor of consider- 

 able importance in their history. 



The associated diagnostic minerals, including the diamonds, 

 are arranged with reference to hardness in the following tables : 



Table showing the Hardness of the Minerals associated 



with Diamonds in Bahia. 

 Minerals Hardness 



J. Diamonds, carbonados 10 . 



2. Corundum 9 



3. Beryl, spinel 8 



4. Zircon 7 - 5 



5. Cyanite, staurolite, cassiterite, diaspore 7 



6. Tantalite 6*5 



7. Brookite, columbite 6 



8. Monazite, octahedrite, titanite, "favas".. 5-5 



9. Xenotime . 5 



The hardness of the diamond (and carbonado) is a strong 

 point in favor of the theory of the possibility of its having 

 been passed down through several geologic ages, but the 

 chances of the survival of a mineral having a hardness below 

 6 - seems rather small though not at all impossible. The value 

 of this evidence, therefore, seems to be doubtful. 



Conclusion. — It is fully realized that the results of this 

 study are mostly negative. There is no evidence, however, 

 that the Brazilian diamonds are of eruptive origin. And they 

 certainly were not brought into their present position in the 

 Lavras quartzites by eruptive action of any kind. 



I am strongly inclined to believe that the diamonds of Bahia 

 have their origin in the quartzites where they have been occa- 

 sionally found in place, and that they are associated with min- 

 erals characteristic of metamorphic rocks for the reason that 

 those minerals also originated in the quartzites under the 

 same conditions as the diamonds themselves.* 



*The occurrence of diamonds in place in South Africa and in Arkansas, 

 and associated in both instances with peridotites, appears to have thrown 

 the burden of proof ujDon any geologist who ventured to suggest any other 

 than an igneous origin for diamonds in other parts of the world. But even 

 this stronghold of the believers in the igneous origin of diamonds is seriously 

 shaken by two important facts: — 



First, that diamonds are found in only a small percentage of the South 

 African eruptive pipes. 



Second, that diamonds have been found in the garnets of the eclogites 

 brought up into the diamond-bearing pipes by the eruptives. So that after 

 all, a metamorphic rather than an eruptive origin of even the South African 

 diamonds seems to be a possibility, if not a probability. 



[A. L. du Toit. — The diamond-bearing blue-ground and allied rocks of 

 South Africa, Trans. Edin. Geol. Soc, lx, 361-362, Edinburgh.] 



