Elasticity Matrix for Silicon

Hi all,

I wonder would someone be able to shed light on something for me.

In Si material properties, the anisotropic elasticity is defined as a 6x6 matrix:
{D11, D21, D31, D41, D51, D61, D12, D21, D22, D23 ... and so on up to D66} .

However, the expression written into the 6x6 matrix is as below only has 21 values:
{166[GPa], 64[GPa], 166[GPa], 64[GPa], 64[GPa], 166[GPa], 0[GPa], 0[GPa], 0[GPa], 80[GPa], 0[GPa], 0[GPa], 0[GPa], 0[GPa], 80[GPa], 0[GPa], 0[GPa], 0[GPa], 0[GPa], 0[GPa], 80[GPa]}

Why are 36 values not written into the expression? I just want to check that the elasticity matrix agrees with the literature.

Thanks for reading, hopefully someone can help.

Haydn