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Volume 6, pages 88-90, 1921
The old cobalt mine in Chatham is one of the many diverse types of metal deposit in New England which have in times past enticed speculative persons into spending money in their development without ever repaying the money invested, yet giving to mineralogical science numerous valuable specimens. The region about Cobalt was believed to contain rich mineral deposits from the earliest times. There is an old legend to the effect that Governor Winthrop was accustomed to repair to his mines here for a certain period of each year with a single henchman and to secretly mine, smelt and manufacture gold to supply himself for the balance of the year.
Dana lists from here the minerals arsenopyrite, smaltite, chloanthite (chathamite), scorodite, niccolite, and erythrite. "Chathamite," named for the locality, is an iron-rich chloanthite. The name is one of Shepard's. An analysis of this mineral by Genth gave: As 70.11, S 4.78, Co 3.82, Ni 9.44, Fe 11.85, sum 100 per cent.
The vein seems to strike about due east-west and to dip with the inclosing rocks about 45° to the north. The most
abundant rock is a dark sandy quartz gneiss containing thin laminas of coarser foliated biotite and garnet. The "chathamite"
occurs in small grains, bunches and veinlets intergrown with the biotite in these biotite-rich layers. The blocks of
this rock containing the cobalt minerals are stained greenish yellow on the outside and are rolled and crumpled. They
are very tenacious and emit an arsenical or sulfurous odor when broken, even when no cobalt minerals are visible.
Associated with the biotite quartz gneiss is some silvery muscovite or sericite schist which contains no metallic
minerals. In one place a small amount of a much altered basic igneous rock was seen. The vein seems to be merely a zone
in the gneiss where the cobalt minerals occur in disseminated form. This zone contains quartz veins and lenses of two
types. The first type is composed of granular quartz rendered schistose by thin parallel partings of mica, and contains no cobalt or other metallic minerals. The second type is coarsely crystalline white quartz, often translucent
and containing abundant large crystalline grains and aggregates of "danaite." Where these bunches of
cobaltiferous arsenopyrite are decomposed they yield earthy aggregates of a greenish yellow to gray color which may
include scorodite and erythrite. No niccolite was found in the short time spent at the mine.
The buildings which were used in connection with the mining have vanished and such underground workings as were opened are also lost to sight. The dumps are still accessible, however, and yield abundant specimens of the above described minerals.
1 Loomis, I. F., The Town of Chatham, Conn., Quart.. 5, 370, 1899.