  We present in this work measurements performed with a small Micromegas-TPC
using a xenon-trimethylamine (Xe-TMA) Penning-mixture as filling gas.
Measurements of gas gain and energy resolutions for 22.1 keV X-rays are
presented, spanning several TMA concentrations and pressures between 1 and 10
bar. Across this pressure range, the best energy resolution and largest
increase in gain at constant field (a standard figure for characterizing
Penning-like energy transfers) is observed to be in the 1.5%-2.5% TMA region. A
gain increase (at constant field) up to a factor 100 and a best energy
resolution improved by up to a factor 3 with respect to the one previously
reported in pure Xe -operated Micromegas, can be obtained. In virtue of the
VUV-quenching properties of the mixture, the overall maximum gain achievable is
also notably increased (up to 400 at 10bar), a factor x 3 higher than in pure
Xe. In addition, preliminary measurements of the electron drift velocity in a
modified setup have been performed and show good agreement with the one
obtained from Magboltz. These results are of great interest for calorimetric
applications in gas Xe TPCs, in particular for the search of the neutrino-less
double beta decay of Xe-136.
