How different would the internal composition of a carbon planet be from that of Earth?

My idea is a carbon planet 50 % larger than Earth and with 5 times its mass (approximately), but I set these values ​​after investigating those of several discovered exoplanets and none of them is considered a carbon planet, they are probably more similar to Earth in terms of their internal composition.

I understand that the planets of carbon could have a mantle of carbides instead of one of silicates as with Earth; and if they are massive enough, they could have a diamond layer near their core. Would there be any significant change with the mass of the planet considering an internal composition like this? I mean, I understand that the mass of exoplanets is estimated considering an internal composition like that of the Earth, but the fact that the planet has, for example, a mantle of carbides instead of one of silicates, would the mass be different?

• Are you asking about mass estimation based on measured radius? – Arkenstein XII Aug 14 at 3:57
• That's right. Also considered the influence of the internal composition (carbon or silicon), if any. – URIZEN Aug 14 at 4:08
• Generally, volume scales as radius cubed ($r^3$). Assuming the same density, mass scales with volume. Replacing Si with C, the mass ratio is 12:28, based on atomic masses, so about half the mass. In your case, $1.5^3*12/28$ times Mass of the Earth should give a rough approximation – nzaman Aug 14 at 6:21