  We follow-up on a previous finding that AGB Mira variables containing the
3DUP indicator technetium (Tc) in their atmosphere form a different sequence of
K-[22] colour as a function of pulsation period than Miras without Tc. A near-
to mid-infrared colour such as K-[22] is a good probe for the dust mass-loss
rate of the stars. Contrary to what might be expected, Tc-poor Miras show
redder K-[22] colours (i.e. higher dust mass-loss rates) than Tc-rich Miras at
a given period. Here, the previous sample is extended and the analysis is
expanded towards other colours and dust spectra. The most important aim is to
investigate if the same two sequences can be revealed in the gas mass-loss
rate. We analysed new optical spectra and expanded the sample by including more
stars from the literature. Near- and mid-IR photometry and ISO dust spectra of
our stars were investigated. Literature data of gas mass-loss rates of Miras
and semi-regular variables were collected and analysed. Our results show that
Tc-poor Miras are redder than Tc-rich Miras in a broad range of the mid-IR,
suggesting that the previous finding based on the K-[22] colour is not due to a
specific dust feature in the 22 micron band. We establish a linear relation
between K-[22] and the gas mass-loss rate. We also find that the 13 micron
feature disappears above K-[22]~2.17 mag, corresponding to $\dot{M}_{\rm
g}\sim2.6\times10^{-7}M_{\sun}yr^{-1}$. No similar sequences of Tc-poor and
Tc-rich Miras in the gas mass-loss rate vs. period diagram are found, most
probably owing to limitations in the available data. Different hypotheses to
explain the observation of two sequences in the P vs. K-[22] diagram are
discussed and tested, but so far none of them convincingly explains the
observations. Nevertheless, we might have found an hitherto unknown but
potentially important process influencing mass loss on the TP-AGB.
