Structil is a brand name for carbon tubes. The standard weight tubes are similar to the Avia pultruded tubes available here. .2300 is generally the best equivalent to a 6mm tube.
I think that ø 6 mm is .2400" and ø 5.9 mm is .2300"
Structil is a bit stiffer and lighter than Avia tubes.
The weight for Structil spars for 1 meter in g/m were given to me some years ago by the Structil Company :
W = ([D + d] x [D - d]) x 1.255 <- weight in grams/ meter
That means for example for a ø 6 mm O.D. diameter and ø 4 mm I.D. diameter tube : ([6 + 4) x [6 - 4)]) x 1.255 = 20 x 1.255 = 25,1 g/m
Structil also makes a high modulus product which is fairly unique these days.
Hi-modulus Structil were very good spars but they are no more available.