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We present the observations of a weak MgII absorption system detected atz~0.452 in the UVES high resolution spectrum of the QSO HE0001-2340. The weakerof the two MgII components forming the system shows associated absorptions dueto SiI, CaI and FeI observed for the first time in a QSO spectrum. Weinvestigate the nature of this absorber by comparing its properties with thoseof different classes of absorbers (weak MgII, Damped Ly-alpha systems and localinterstellar clouds) and reproducing its ionization conditions withphotoionization models. The observed absorber belongs to the class of weak MgIIsystems on the basis of its equivalent width, however the relative strength ofcommonly observed transitions deviates significantly from those of the abovementioned absorbers. A rough estimate of the probability to cross such a systemwith a QSO line of sight is P~0.03. The presence of rare neutral transitionssuggests that the cloud is shielded by a large amount of neutral hydrogen. Adetailed comparison of the observed column densities with the averageproperties of damped Ly-alpha systems and local interstellar cold clouds shows,in particular, deficient MgII/MgI and CaII/CaI ratios in our cloud. The resultsof photoionization models indicate that the cloud could be ionized by the UVbackground. However, a simple model of a single cloud with uniform densitycannot reproduce the observed ionic abundance ratios, suggesting a more complexdensity structure for the absorber. Supposing that ionization corrections arenegligible, the most puzzling result is the underabundance of magnesium withrespect to iron which is hard to explain both with nucleosynthesis and withdifferential dust depletion. [Abridged]

A cold metal-poor cloud traced by a weak MgII absorption at z~0.45. First detection of SiI, CaI and FeI in a QSO absorber