Implement Kruskal's algorithm in python to find a minimum spanning tree. Input and output should be in the form of an adjacency matrix.

Example, kruskal(G):

Input in Matrix representation:

G = [[0, 9, 75, 0, 0],
[9, 0, 95, 19, 42],
[75, 95, 0, 51, 66],
[0, 19, 51, 0, 31],
[0, 42, 66, 31, 0]]

Sample Output in matrix representation:

G = [[0, 9, 0, 0, 0],
[9, 0, 0, 19, 0],
[0, 0, 0, 51, 0],
[0, 19, 51, 0, 31],
[0, 0, 0, 31, 0]]