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ct-Ca3(PO4) 2 crystallizes in the monoclinic space group P2Ja with a= 12.887(2), b=27.280(4), c--15.219 (2) ,~,/~ = 126.20(1) °, and Z = 24. An approximate subcell with b' = b/3 exists. The structure was solved by direct methods of phase determination and refined to R = 0.051 and R,= 0.049 with 7002 reflections collected from a single crystal by counter methods. The structure consists of columns of cations, and columns of cations and anions with compensating cation vacancies, and is closely related to the structure of K3Na(SO4) 2, the mineral glaserite. The calcium ions in ct-Caa(PO4) 2 exhibit a wide range of coordination numbers and geometries.

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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