What is the composition of an asteroid in percentages?
You might find the information you seek in this article by Nature on the compositional mass distribution of asteroids.



From Wikipedia Asteroid Spectral Types:


  The majority of bodies fall again into the three broad C, S, and X
  categories, with a few unusual bodies categorized into several smaller
  types:

C-group of carbonaceous objects including:[citation needed]
    B-type largely overlapping with the Tholen B and F types.
    C-type the most 'standard' of the non-B carbonaceous objects
    Cg Ch Cgh somewhat related to the Tholen G type
    Cb transition objects between plain C and B types.
S-group of silicaceous (stony) objects including:
    A-type
    Q-type
    R-type
    K-type a new category (181 Eucharis, 221 Eos)
    L-type a new category (83 Beatrix)
    S-type the most 'standard' of the S group
    Sa, Sq, Sr, Sk, and Sl transition objects between plain S and the other types in the group.
X-group of mostly metallic objects including:
    X-type the most 'standard' of the X group including objects classified by Tholen as M, E, or P-type.
    Xe, Xc, and Xk transition types between plain X and the appropriately lettered types.
T-type
D-type
Ld-type: a new type with more extreme spectral features than the L-type
O-type a small category (3628 Božněmcová)
V-type (4 Vesta)



You will have to go to the Wikipedia article to drill down for more detail on the probable minerals contained in these asteroid classes.  We really only have good evidence on those classes that we think we have meteor samples of.  The rest would be guesses.