Arago nit e (C aCO ,,) c ry s ta lli zes in th e unit ce ll a = 4.9598(5) /\ . 6 = 7.9641(9) A, and c = 5.7379(6) A at 25 °C with four form ula we ight s in s pace-g roup Pm cn. Th e s tru cture has bee n re fin ed to N".= 0.024 . N = 0.040 using 765 x-ray re Aec tion s fro m a sin gJe c rysta l. Th e Ca ion is coo rdinat ed to nin e oxygens with Ca .. . ° di s tan ces III th e range 2.414(2) A to 2.653(1) A. Th e t wo un iqu e C -O di s ta nces in th e CO" group a re 1.288(2) ;\ (o n th e mirror plan e) a nd 1. 283(1) A. T he two un iqu e O-C-o a ngles are 119.6(2)" (ac ro ss th e mirror plan e) and 120.1 3(8)". Th e d is tan ces a nd an gles in th e CO" "ro up are not signifi ca ntl y d iffe re nt a t th e 95 pe rce nt co nfid e nce leve l. "Key word s; Ca lc ium carbonat es; ca rbonates; crysta l s tru c ture; s in gle c rys t.al x-ray diffrac ti o n.

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Ca(H2PO4)2. H20 crystallizes in space group PI in the triclinic unit cell a= 5.6261 (5), b = 11.889 (2), c = 6-4731 (8) ~, ~ = 98"633 (6), fl= 118.262 (6), and 7' = 83.344 (6) ° at 25 °C with Z= 2. The structure was refined with 3401 observed reflections measured on a diffractometer to Rw = 0-033, Ro = 0.069. Corrections were made for absorption, anomalous dispersion, and isotropic secondary extinction. The structure contains Ca(H2PO4) + chains which form corrugated layers. Between these layers are layers of (H2PO4)-ions and water molecules. Hydrogen atoms have been located approximately; ideal positions for the hydrogen atoms have been calculated. The two sets of hydrogen positions differ by ~ 0.28 ,&. One hydrogen bond, in which the water molecule is the donor, is apparently bifurcated. All other hydrogen atoms form normal hydrogen bonds, with each hydrogen site seemingly fully occupied. The disorder of one hydrogen atom between the centrosymmetrically related atoms O(1) and O(1') was postulated by earlier workers. The P-O and O...O distances and electron-density maps obtained here show that there is no hydrogen atom in this position.

Th e barytoc al c ite ph ase of BaCa(CO,,), c rys ta llizes in th e monoclini c unit ce ll a = 8 .092(1) A, b = 5.2344(6) A, c= 6.544(1) A, (3 = 106.05(1t a t 25 °C with ce ll conte nts of 2[B aCa (C O ,,), I. Th e Slru et ure pre viou s ly re ported b y Aim is co rrec t in its coa rse d etail s but has bee n rede te rmin e d he re and re fin ed to R w = 0 .028 , R = 0.023 in s pa ce-gro up P2 1 / m usin g 1652 o bse rved re Rections. Co rrec ti ons we re made for abso rpti on, isotropic ex tin c ti on , a nd a no ma lo us di s pe rs io n.Th e stru c ture of ba ryt oca lc ite h as a n ... AB C ABC ... s tac kin g of ca ti on layers a nd re pe at e ve ry 3 la yers . Th e c al cite phase of CaCO:1 ha s a n AB C c ati on layer se qu e nce a nd re peats e ve ry 6 laye rs. Th e orie ntati ons of th e CO:1 gro up s in bar ytoca lcite a re lik e th e CO :1 gro up ori ent ati o n in th e a rago nit e phase of CaCO:1, and are rot ate d a bo ut 30 ° from t he CO " gro up ori e nt a ti on in ca lcite. Th e cation laye r sequ e nce in a ragon ite is.. ABAB.. a nd th e s tru c ture re pea ts e ve ry 2 la ye rs. Th e Ca ion in baryto ca lc it e is coordin at,e d to se ve n o*ygens, in c lu din g a n edge of a CO" group , with Ca .. . ° di s tan ces in th e ra nge 2. 305(2) A to 2.5 18(2) A. Th e Ba io n is coord in a te d to fiv e ed ges a nd on e apex of th e CO:1 gro ups with Ba . .. ° di s tan ces ra ngin g from 2.729(3) A to 3. 14;0(2) A. Th e di~ l a nces of th e C atom s in th e CO :1 ~r o up s fro m th e pl a nes of th e ° a toms a re 0.025(5) A a nd 0.022(4) A for C(l ) and C(2), res pec tivel y.Key word s : Aragonite ; barium ca lc ium ca rbonate; cal c ium carbonate; c r ys ta l stru cture; s in gle c r ys ta l x-ra y diffrac tion.

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