In most arsenic minerals, the arsenic replaces sulphur in more familiar minerals. The hard and heavy mineral NiAs, niccolite or kupfernickel, is an example. Silver white cobaltite, CoAsS, is hard and heavy and occurs in pyritohedrons, but is distinguished from pyrite by its colour. FeAsS, arsenopyrite, is perhaps the commonest arsenic mineral, and is also called mispickel. It is found in striated prisms. The mineral once known as smaltite, now as skutterudite, is (Co,Ni,Fe)As3; it contains varying quantites of the iron group metals. Proustite, Ag3AsS3, or "ruby silver," is soft and of medium weight. Cu3AsS4 is black enargite, the source of arsenic in the mines of Butte, Montana. Note that in the last two compounds, S has replaced O in the arsenate and arsenite. Enargite is cuprous thioarsenate.

Maybe not "arsenic metal" but may indeed form deadly compositions and be deadly. Arsenic metals have a better capability to absorb into human tissue and cause serious damage due to polarity changes of arsenic. Nickel itself is quite dangerous itself.