Extrasolar Refractory-Dominated Planetesimals: an Assessment
  Previously published observations of 60 externally-polluted white dwarfs show that none of the stars have accreted from intact refractory-dominated parent bodies composed mainly of Al, Ca and O, although planetesimals with such a distinctive composition have been predicted to form. We propose that such remarkable objects are not detected, by themselves, because, unless they are scattered outward from their initial orbit, they are engulfed and destroyed during the star's Asymptotic Giant Branch evolution. As-yet, there is at most only weak evidence supporting a scenario where the composition of any extrasolar minor planet can be explained by blending of an outwardly scattered refractory-dominated planetesimal with an ambient asteroid. 
M. Jura, S. Xu
astro-ph.EP