JONI Mitchell's 'Mingus' (Asylum 5E-505) is not only the most important record she has made, it also marks a significant crossover.

The album is the result of melodies written for Joni by the great bassist Charles Mingus, who died last January aged 56. Mingus wanted Joni to write and sing the lyrics on a collaborative album but he died before he could hear the final product.

It might at first seem an unlikely combination: the black jazzman, burning with racial anger, and the white middle class pop-folk singer. But Mingus always had reached toward new boundaries; Joni has developed a close rapport with jazz and her lyrics are often more social poetry than pop words.

The musicians on Mingus represent the new jazz generation and are mostly familiar on Joni Mitchell albums: Jaco Pastorius (bass), Wayne Shorter (soprano sax) and Peter Erskine (drums) of Weather Report, plus Herbie Hancock (electric piano), Don Alias (congas) and Emil Richards (percussion).

There are six melodies, plus some "rap" sessions recorded earlier with Mingus and friends. Two tracks are Joni's compositions.

The first, God Must Be A Boogie Man, is a wry tribute to Mingus that contains some harsh comment:

"Blind faith to care
Blind rage to kill..."

Joni's sharp acoustic guitar chords punctuate the free-swinging refrain of vocals and Jaco's mournful bass.

A Chair In The Sky is a slow, bluesy Mingus ballad, in which Joni's voice and Shorter's sax blend at times in uncanny unison.

The Wolf That Lives in Lindsey is another all-Joni track, complete with gongs and wolves eerily howling.

Sweet Sucker Dance swings along with the merest brush of percussion as Joni lilts through the airy melody, using her voice in what might have been a horn lead, scatting at times. The rhythm hardens as the track progresses.

The Dry Cleaner From Des Moines has a rocking Caribbean pace, driven appropriately by Pastorius's bass. He arranged the dynamic brass sections. Over the top come Joni's voice and Shorter's sax, singing together.

Goodbye Pork Pie Hat is Mingus' memorable blues tribute to Lester Young. Here it is a free-singing, slowly rhythmic tribute to both jazzmen, with piano, bass, percussion and breathy sax - an atmospheric setting for Joni's lyrics.

'Mingus' should appeal to fans of Joni Mitchell, Weather Report and Mingus himself...perhaps they now belong to the one group.

Mingus influenced and significantly changed many top musicians. Joni Mitchell may now number among them. As she told an interviewer recently:

"After this, rock 'n' roll is like a metronome."