Sepiolite asbestos from Franklin, New Jersey: a case study in medical geology.

Asbestiform sepiolite has been found in a zinc deposit at Franklin, New Jersey. The host rock is Precambrian Franklin marble. Pseudomorphous foliated texture and cross-cutting relationships indicate replacement of talc by sepiolite. Sepiolite is manganoan, while talc contains little manganese, suggesting differences in manganese substitution in these minerals and providing evidence against solid-state replacement. Sepiolite from Franklin is of moderate crystallinity and consists of soft, flexible mass-fiber. Comparison with other sepiolite samples suggests that increased crystallinity among sepiolites parallels increased fiber length, while disorder appears to be associated with flexibility. Length and aspect ratio (length/width) distributions support toxicological equation of asbestiform sepiolite with the regulated asbestos minerals. The data presented here suggest that talc can be unstable in a low-temperature hydrothermal environment, altering under certain conditions to form sepiolite. In a wider context, sepiolite may be expected to precipitate at the end-stages of hydrothermal mineralization in some talc deposits and in carbonate and calc-silicate rocks. Many occurrences of sepiolite in these rocks may have been overlooked due to similarities with other minerals. The fact that sepiolite has been previously unrecognized and misidentified at Franklin is a case in point. Since there is evidence suggesting that sepiolite is fibrogenic and carcinogenic, its potential occurrence in consumer talcs and crushed-stone products has environmental health implications. Similarities in composition and other analytical parameters may cause sepiolite to be mistaken for fibrous talc or chrysotile in environmental samples.