Priaulx Rainier has become a footnote in other people’s lives, and sometimes not even that. A South African-British composer whose life (1903-86) spanned much of the twentieth century, she wrote two concertos for Yehudi Menuhin, who pronounced her to have a ‘musical imagination of a colour and variety scarcely to be believed.’ Yet in Menuhin’s biography ‘Rainier, Priaulx’ appears nowhere in the index. The eye often travels down the ‘R’ column of indexes, looking for her in vain above the Rambert ballet or ‘Rattle, Sir Simon’.

But occasionally it finds her: cycling through the rubble of Blitz-crushed London in the company of Lucien Freud, or sitting in the garden of the sculptor Barbara Hepworth’s Cornish studio. Here she is in a café in Paris collaborating with the Surrealist poet David Gascoyne; here in a smart London restaurant being introduced to her hero, Igor Stravinsky, to whom she demonstrated the click consonants of Xhosa. She wrote pieces for the tenor Peter Pears and for the cellist Jacqueline du Pré, and became so close to the composer Michael Tippett that, although both were gay, they considered a Platonic marriage. Others remembered her wise beauty, her melodious speaking voice, her habit of snorting when she laughed, and the eccentric but delicious way she made coffee, flinging ground beans onto hot milk.

Her mother was English, her father Huguenot in origin, and she was born and raised in what was then the British colony of Natal, in South Africa. Childhood’s soundscape loomed large in the music she went on to write: Xhosa clicks, the chirrup of insects, the bang of electrical thunderstorms. A gifted violinist, she came to London when she was seventeen to study at the Royal Academy of Music, and although she stayed in Britain for the rest of her life she never quite lost the sense of being a foreigner abroad. A car accident left her unable to play and she turned to composition. Praise from the composer Arnold Bax was an encouragement, and lessons in Paris with conductor Nadia Boulanger were transformative, although hers was a laborious apprenticeship: music did not come easily, and she wrote nothing with which she was satisfied until her mid-thirties.

During the war Rainier was appointed a professor at the Royal Academy (she was a gifted and inspiring teacher, and in Tippett’s opinion one of the greatest in London). She became an air-raid warden in Kensington and then a land-worker in Hertfordshire, forking hay. Always happiest in the company of artists, she befriended Henry Moore and John Craxton. With Tippett’s support she found a publisher and a small number of performances. William Glock, music critic of the Observer, thrilled to her Bartók-tinged String Quartet, written in 1939, and given its first public outing in 1944: ‘I cannot recommend this work too strongly … Had Miss Rainier written nothing but the first movement, she would still be one of our best composers.’ The quartet’s nagging clockwork, its wisps of melody wrapped in a protective film of pizzicato, show her seemingly innate ability to produce new sounds from conventional forces.

After the war she divided her time between a cheap flat in London’s Notting Hill (such things existed, then) and a studio in the light-saturated Cornish town of St Ives, which juts out into the Atlantic. There she became an integral part of the town’s artists’ colony and an intimate friend of sculptor Barbara Hepworth who had moved there at the outbreak of war. Rainier would compose in a small shed near Hepworth’s studio, and is responsible for much of the landscaping of the famous garden, in which Hepworth’s sculptures still sit in quiet communion with sea and plants. Rainier’s Rhythms of the Stones is a sonic portrait of Hepworth and her assistants at work on the sculpture Contrapuntal Forms, notating the rhythm of hands and tools as they struck Irish Blue Limestone.

Rainier’s conception of composition became increasingly sculptural, architectural; she hewed music out of sound or welded together great blocks of material. Her consideration of music was often visual, and she frequently discarded the developmental in favour of overlaying static fragments to form into what she once called ‘honeycombs’ of sound. Elsewhere she spoke of bunching together musical atoms, or allowing them to repel one another, to be left spinning in air.

Hepworth’s sculptures, meanwhile, began to take titles such as Rhythmic Form, and Rainier gifted her Tippett’s recording (the first) of Thomas Tallis’s forty-part motet, Spem in Alium: ‘it is right that you should have the score of this miracle of architecture in sound.’ Hepworth set to work: ‘I’m determined to understand something of the construction.’ She wrote to a friend: ‘Priaulx Rainier is staying with us. She is a very beautiful person with a rare intelligence and astonishing creative power.’

It was with Hepworth and Tippett that Rainier co-curated an arts festival in St Ives to celebrate the coronation of Queen Elizabeth, in 1953. Administration was not her strong suit, and the festival, while it brought together renowned performers and artists, had more than its fair share of disasters: a concert of madrigals given from a boat in the harbour was abandoned when it transpired that Rainier had not remembered to check the tides. The singers became less and less audible as the boat drifted out to sea. Only gradually did she achieve success, but when it came it was, for a time, considerable. She was championed at the Proms and by the BBC, which recorded and broadcast her complete chamber music, in 1976.

But her published catalogue runs to fewer than thirty works and her pieces are often small-scale, though among her orchestral music, carbonated with percussion and blaring with visceral attack, are the two concertos for Menuhin; a cello concerto for du Pré; and an entrancing suite called Aequora Lunae, each section depicting volcanic eruptions on the moon’s surface that astronomers once thought to be seas. Such music admits few British influences (Elgar, say, or Vaughan Williams). Her god was Stravinsky: she was indebted to the rhythmic wildness of his early ballets, to the sonorities and ostinatos of his neo-classical phase, and to the crystalline sound and inventive orchestration of his late, serial, works. She had a clear appreciation for, but never adopted, twelve-tone or serial technique, and instead developed a voice that could be radically and experimentally tonal, a voice of discords, dissonance and chromaticism in which major triads can suddenly appear new minted and, given their surroundings, sonically shocking.

Critics groped for words such as ‘tough’ and ‘uncompromising’ to describe what was perceived as an almost brazen lack of interest in melody, but her music contains a lyricism that captures the light in a startlingly beautiful way, such luminosity all the more striking for glinting from darker shards. She could be dismissed with casual sexism – William Walton was certain that such a composer must have worn ‘barbed-wire underwear’ – but, maddeningly vague and apparently joyous company, she inspired personal loyalty and professional admiration. Her output, wrote William Glock, was a ‘model of fearless individuality in which one feels that not a note could be other than it is … a composer of integrity and vision.’ A defining characteristic of her music is its ability to create, often with very small forces, a sense of light and space in, and within, sound. Most distinctive of all is her persistent use of precisely notated silence.

Rainier perfected the art of the epic miniature. Cycle for Declamation (1954), a three-part setting of John Donne for unaccompanied tenor, shows her astonishing and uncompromising way with the human voice. Peter Pears, the first performer, had to navigate a wild vocal line, made to stutter and judder by oddly placed rests, but as frequently allowed to unwind into long lyrical ribbons, single syllables stretched across extended chromatic phrases. The slow, central, panel in the triptych is an icy lake of melody between the outer movements’ pebbled shores.

The Bee Oracles (1969) sets Edith Sitwell’s poem ‘The Bee-Keeper,’ the tenor accompanied by the novel buzz of flute, oboe, violin, cello, and harpsichord. A central section, the calm centre of the piece’s flurry, achieves an especially luminous quality, the soloist threading his line above a slowly revolving instrumental shimmer, well ventilated with rests.

Today she is barely remembered and few of her pieces are recorded. She was survived for only a few years by her partner, June Opie, and left no heirs to champion her cause. Her minimal royalties are given to charity. But to ignore her is to ignore a major influence on the art of Barbara Hepworth and, especially, Michael Tippett, who often professed his debt to Rainier’s music. Like Tippett, she had little natural facility and composition was often a slow and arduous, even dogged, process, challenging the notion of ‘technique’ as something absolute, either present or absent. Rainier worked out a technique for each piece as she went along and her scores would arrive at her publishers full of miscalculated beats. But her music is alive with an energy born from the tension between the sounds she heard in her head and the marks she put on manuscript paper that they might be recreated.

She died unexpectedly, of a heart attack, on a holiday in the French Alps with Opie. A few days before her death the couple had driven up through the mountains to explore a series of extinct volcanoes. Rainer had walked off alone – ‘I’m going to listen to the silence’ – and had stood on the edge of a crater, her trousers rolled to her knees, her head thrown back, listening.