Summary: Vaterite, the third unstable polymorph of CaCO₃, is recognized by optical and X-ray powder techniques as a major constituent in a carbonated calcium silicate hydrogel complex formed from larnite at Ballycraigy, Lame, Northern Ireland. Re-indexed X-ray powder data obtained on synthetic vaterite are provided indicating a hexagonal unit cell with dimensions a = 7·135 Å. and c = 8·524 Å., Z = 6CaCO₃. The re-indexing is endorsed by the results of electron diffraction study of synthetic vaterite. Vaterite is shown to correspond to the hypothetical CaCO₃ member of the bastnäsite-synchisite mineral series on the basis of extrapolated cell dimensions and optical properties. A structural scheme for vaterite is proposed in which Ca layers replace the CeF layers in bastnäsite, and near-vertical CO₃ groups are present. The stability relations of natural vaterite are discussed and comment made on the recent observation of vaterite, in a like environment, in carbonated cements and mortars.