Serpentinized ultramafic rocks of Aphebian age occur along a northeasterly trending zone of complexly faulted gneissic rocks in the Setting Lake-Moak Lake region of central Manitoba. The predominant serpentine mineral in these rocks is a fibrous variety which can only be satisfactorily indexed on the basis of a 3-layer structure. The derived 3-layer orthorhombic, pseudohexagonal cell has dimensions of a=5.346 A, b=9.205 A and c=21.93 A. Antigorite and lizardite from the area have also been identified by optical X-ray diffraction and differential thermal analysis methods. Additional occurrences of antigorite from Oxford Lake, Carrot River, Knee Lake and Island Lake in Manitoba and Lancaster County in Pennsylvania are described and compared with the Caracas antigorite.

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