Sonja Kristina of Curved Air fits in this category of female vocalist i once blogged about as That Voice...  a type exemplified by Grace Slick, Dorothy Moskowitz of The United States of America, and Lynn Carey who did the lead voice for the Carrie Nations songs on the film soundtrack of Beyond the Valley of the Dolls, but for the actual soundtrack album itself was replaced by another female singer who wasn't quite as powerful

"That Voice" meaning a vocal style that was white and strong (or maybe strident is the word) ... clear, sustained, piercing...   not obviously influenced by rhythm-and-blues or soul or jazz, but more like a mixture of folk and show tunes that adds up to something else altogether.... a hint of witchy mystery...  feminine but forbidding...

Sonja K was a folky sort of singer but she'd also been in Hair

(when i was reading old music papers researching Rip It Up, i remember a swipe at Siouxsie and the Banshees saying that they were just Curved Air with a different hair cut -- but that seems like a cheap and also inaccurate shot)

the Continent is probably full of That Voice candidates -- Mariska Veres of Shocking Blue (especially on "Love Buzz")...   Edna Béjarano of The Rattles as in "The Witch"...  Annisette Hansen in Savage Rose

Nico would be That Voice but her voice isn't strong enough in pure power terms.... there has to be a sense of formidable and imposing and a little bit intimidating, even...

Stevie Nicks is almost in the zone but a  tad too husky, too rock'n'roll gritty..

More Sonja K!

The lead singer in Heart approaches it, via a transgendering of the already epicene Robert Plant

Here's some further thoughts I had on this subject on blissblog back in the day, duplicating some of the very same points made above! if nothing else the way my brain works is a consistent process...