a mixed dislocation
The pure edge and the pure screw are the two tidy extremes, but a real dislocation is rarely so obedient. Follow a dislocation as it curves through a crystal — most of them are loops or wiggling lines, not perfectly straight — and you find that at one place the line runs perpendicular to its Burgers vector (edge there), at another it runs parallel (screw there), and everywhere in between it sits at some slanting angle. A mixed dislocation is one whose line makes a general angle with its Burgers vector, so it is part edge and part screw at once.
The precise idea rests on one fixed fact: the Burgers vector b is the SAME everywhere along a single dislocation line — it is conserved. What changes as the line curves is the line's own direction. So resolve b into two pieces relative to the local line: the component of b perpendicular to the line is the edge part, and the component parallel to the line is the screw part. If the line makes angle theta with b, the edge component has size b sin(theta) and the screw component b cos(theta). At theta = 90 degrees it is pure edge; at theta = 0 it is pure screw; in between it is genuinely mixed, blending both stress fields.
This is why a single closed dislocation loop lying in its slip plane is so instructive: because b is fixed but the line sweeps through every direction around the loop, one arc of the loop is pure edge, the arc a quarter-turn away is pure screw, and the rest is mixed. The whole loop expands under stress because every segment glides, each in its own way. Mixed character is the rule, not the exception — pure edge and pure screw are the clean idealisations we analyse first precisely so we can understand the mixed reality.
A dislocation loop in an FCC crystal has one Burgers vector, say b along [10-1]. Where the loop line runs along [10-1] it is pure screw (b parallel to line); a quarter of the way around, where the line runs along [1-21] (perpendicular to b within the slip plane), it is pure edge; the four connecting arcs are mixed, splitting b into edge and screw shares according to the local angle.
Along one loop with a single Burgers vector, character shifts continuously from edge to screw and back — mixed everywhere in between.
Mixed does not mean the Burgers vector changes — b is constant along the whole dislocation. It is the ANGLE between the line and the fixed b that varies, and with it the edge/screw split.