  (abridged) The physical mechanisms that make a neutrino with standard-model
(SM) weak interactions a "lepton-number conservation (LNC) violating" neutrino
such as the Majorana neutrino are analysed in a basis of two Majorana states
that have opposite charge-parity ("charge-parity basis"). A small Majorana mass
that is larger than any Dirac mass makes the neutrino not a Majorana but a
"pseudo-Majorana" particle that has no definite chirality and therefore has a
different phenomenology than the physical neutrino. A combination of a large
Majorana and Dirac mass of nearly equal value makes the neutrino a Majorana
neutrino. However if this Majorana neutrino has SM interactions, its weak
transition amplitudes squared are a factor 2 smaller than the ones observed for
the physical neutrino. Only with a small Dirac mass that is larger than any
Majorana mass (and in the massless case), the physical neutrino's phenomenology
is correctly predicted by the SM. Such a mass combination makes the neutrino a
Dirac- or (the most likely possibility for the physical neutrino) Pontecorvo's
pseudo-Dirac particle which features neutrino-antineutrino oscillations, that
violate LNC. Pseudo-Dirac neutrinos enable a completely negligible rate for
neutrinoless double-beta decay if there is no Majorana-mass independent decay
mechanism. Off-diagonal components of the mass matrix in the charge-parity
basis make the neutrino a mixture of Dirac field with a different particle and
anti-particle mass (i.e. a mass that violates CPT invariance) and a
pseudo-Dirac field. Such a neutrino leads to a phenomenology similar to the one
with additional generations of sterile neutrinos.
