The strontianiferous aragonite reported in the present paper is an aragonitic variety found in Bayan Obo iron ore deposit,Inner Mongolia. It occurs in veined Nb-，REE-bearing dolomitecarbonatite and is mainly associated with dolomite, ankerite,monazite, carbocernaite, pyrochlore, norsethite,calcite, calcios- trontianite, benstonite, zircon, apatite, phlogopite and albite. Strontianiferous aragonite is similar in chemical composition and crystal texture to aragonite, It acquires this name simply on account of its chemical composition and crystal texture. The mineral occurs as a fine-grained aggregate with individual crystal 0.Ol-0.03mm in diameter, transparent, white in color,luster vitreous,streak white, D=3.34, H(vicies)=180-190kg/mm2 (=3 .8Mohe), K=0.32× 10-6C.G.S.M.cm 3/gram. In transmitted light the mineral is transparent and colorless, cleavage no or very imperfect.It is biaxial negative with 2V=9º， Ng=1.6620, Nm=1.6561, Np=1. 5168. X-ray investigation sho;vs that the mineral is orthorhombic，with unit cell values being a=4.91, b=7.89 and c=5.78,The strongest X-ray spacings are 5.39(5)，3.456(10)，3.354( 8)，3.773(7)，2.398( 7).2.016(9)。1.864( 9). Chemical analysis yields the formula of(Ca0.4964 Sr0.4445 Mg0.0114 Ba0.0170 Al0.0007 Fe0.0038 K0.0009 Na0.0063 Mn0.0016 REE0.0066) 0.9892 [(CO3)0.9973 (OH)0.0308]1.0281 with the brief form being (Ca, Sr)CO3.Its DTA curve and infra-red spectrum are similar to those of aragonite.