Time-integrated optical emission spectroscopy was applied for the analysis of emission spectra, and determination of electron densities and excitation temperatures of basalt plasma induced by 10.6 micron laser radiation. The plasma was studied in air, argon and carbon dioxide, under pressure of 10, 50, and 100mbar. The plasma emission intensity was found to be strongly dependent on the nature of the ambient gas and its pressure. The highest emission intensities and signal to noise ratios were obtained in argon. Depending on the composition and pressure of the surrounding atmosphere, the values of plasma temperature varied between 14,400K (air at 10mbar) and 17,100K (carbon dioxide at 100mbar). Similarly, the electron number density varied between 3x10(16)cm(-3) (10mbar air) and 1.6x10(17)cm(-3) (100mbar CO2). The observed behavior was correlated with the properties of the studied gases, in particular, their mass, thermal conductivity and ionization energy, and the role of the ambient gas in controlling the efficiency of laser-target coupling, laser-plasma interaction and plasma shielding.