I have not yet observed this phenomenon. But there is one thing for sure. It is a gradual arrangement from “bubble-like” cavities subparallel to the c-axis formerly apatite to slender cavities displaying the habit of needle-shaped apatite that is oriented strictly along the c axis. Even if the cleavage of apatite is poorly expressed along (0001) and (0101) the structural element of the first one is still dominant and controls the preferred orientation of older generations of probably apatite (?). Apatite XX pass through different X morphologies from stubby XX at moderate temperature to needle-shaped XX at higher temperature well demonstrated in volcanic rocks. The term foam directs out thoughts to a randomly arrangement of bubbles what is not the case. My guess is that the change from subrounded stubby cavities (low T) to needle-shaped elongated ones marks a pathway from corrosion to high-T precipitation of previously formed apatite XX. While this interpretation is still proven by studies of crystal morphologies any further interpretation of mine would deviate from a sound platform into speculation because I know the host-rock lithologies, e.g., from Portugal but not the regional geology in detail and the minerals associated with apatite. It can only be a small piece of mine in mosaic which has to be puzzled together by the regional expert not an outsider like me.

The illustrated photos may suggest rapid volatile degassing of the monchiquites. The degassing of H2O, Cl, F, and S has been documented in lunar apatites (Apollo 15 and 16). Please refer to Ustunisik et al. (2011) Am. Mineral. 96: 1650–1653 and Ustunisik et al. (2015) Am. Mineral. 100: 1717–1727.

I have not seen yet it but it is too interesting case

This photo is similar to hydroxyapatite!

Despite the beautiful images it is hard to be sure, but for my money the larger features are common inclusions, open at the surface, while the smaller c-axis parallel features are the intersections of fission tracks, i.e. damage trails produced by the fragments of fissioned uranium atoms. That would account for the fact that they are more numerous in the old apatites. They are normally not observed until the sample is etched, but that can occur by natural fluids or by contact with an acid during sample preparation, even acid polishing might be enough. Comparing reflected-light (RL) and transmitted-light (TL) images would give more certainty.