Mrzzy what killed the miners in the pit was probably Carbon Dioxide CO2,( equiv molecular weight of 44) I think Miners call it "choke damp" as opposed to "Fire damp" which I believe is mostly
as follows,
Fire-damp is composed chiefly of methane with varying percentages of other gases. The fire-damp actually given off by coal or other strata has a composition lying between the following approximate limits
Methane from 70 to 98 %
Ethane, propane, etc From 0 to 2 %
Ethylene, acetylene, hydrogen sulphide From 0 to a trace
non flammable components
Carbon Dioxide From 0 to 4%
Nitrogen From 0 to 15%
Oxygen From 0 to a trace
which causes very easily ignited explosive mixtures with air and because of limited air supply underground burns to water and a mixture of Carbon Dioxide and a small proportion of Carbon monoxide is sometimes formed which is Toxic and fatal