The double-a spelling, used certainly to avoid homography with Shanxi (山西 (Shānxī), of shān rather than shǎn), is not a feature of Hanyu Pinyin and cannot be observed elsewhere. It is likely inherited from the pre-Pinyin Sin Wenz system devised and employed by Communist linguists, which was toneless and had designated "irregular spellings" to disambiguate tonally different words in cases where homography was clearly both unavoidable and undesirable. A Sin Wenz textbook from 1938 (《中文拉丁化課本》) explicitly lists the pairs Shaansi (陝西／陕西) and Shansi (山西).
An alternative theory is that the double-a spelling is from the Gwoyeu Romatzyh romanization system, where the third tone is spelled by doubling a vowel, but this is less likely considering the history of Gwoyeu Romatzyh and Sin Wenz, including the historical rivalry between the two systems. (Can this(+) etymology be sourced?)
- A province in China, with its capital at Xi'an and including the Wei River valley and the fertile southern half of the Ordos Loop, comprising much of the Loess Plateau.
- 1984, Pan Jiang (P'an Kiang), “The Phylogenetic Position of the Eugaleaspida in China”, in Proceedings of the Linnean Society of New South Wales, volume 107, page 311:
- Relevant new discoveries in China, not yet described, include polybranchiaspids and hanyangaspids which were recently (1981-82) recovered from an Early Silurian formation in western Hunan Province of south China, and in southern Shaanxi Province, west China.