2.3g is NOT underweight for a
caracalla denarius.
While 3g was the "
standard", you more often see them lower
weight than that, and occasionally much heavier - up to 4g or so.
My lightest
Caracalla denarius right now is 2.21g, but I've
had them as light as 2.09g.
Usually the lighter they are, the more porous they are.
Caracalla's later
denarii are as low as around 50% silver content. The rest is mostly copper, which tends to corrode out near the coin's surface if the coin came from overly acidic soil. This is often why late Severan-era
denarii (and all third century
Roman silvers) sometimes have that "frosted" look. Technically it's called "selective phase corrosion". It's common when you have noble metals alloyed with copper in corrosive environments.
(yes, I have experience of this stuff in my day job).