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Serpentinite from Woods Chrome Mine, Lancaster County, Pennsylvania, consists of planar serpentine, randomly interstratified serpentine-chlorite, a series of phases based on regular interstratification of serpentine and chlorite, minor chlorite, polygonal serpentine, and antigorite. The serpentine mineralogy is complex, including structures with long-range order in (1) the octahedral cation sequence and (2) the sequence of displacements between adjacent 1:1 layers. In addition to previously described (but generally less common) 2T, 2H 1 , 2H 2 , 6R 1 , and 6R 2  lizardite polytypes, the assemblage contains planar serpentines with long-range order in octahedral cation sequences but with random b/3 (and possibly a/3) displacements between adjacent layers. By comparison with calculated electron dif­fraction intensities, we identified three- (I,I,II), four- (I,I,II,II and I,I,I,II), five- (I,II,I,II,II), six- (II,I,I,I,II,II), seven- (I,II,I,II,II,II,II and I,II,I,II,I,II,II), and nine-layer octahedral se­quences. In addition, the assemblage includes rare three- and four-layer serpentines with regular stacking; in some cases the stacking is nonstandard, insofar as zero and ±b/3 displacements occur together. By comparison between electron diffraction intensities and calculated patterns, we identified a three-layer regular stacking sequence that involves 0,0,-b/3 displacements between adjacent layers (α = 98°, β = 90°, γ = 90°), a four-layer monoclinic sequence with 0,-b/3,0,+b/3 (α = 90°, β = 90°, γ = 90°; I,I,II,II), and three four-layer triclinic sequences with 0,-b/3,-b/3,-b/3 (α = 90°, β = 90°, γ = 90°), 0,0,0,-b/3 (α = 96°, β = 90°, γ = 90°), and -b/3,-b/3,-b/3,+b/3 (α = 96°, β = 90°, γ = 90°) displacements between adjacent layers. Planar layer silicates exhibit macroscopic preferred orientation. The strong lineation in the foliation defined by the silicate sheets parallels either a or b, suggesting serpentine crystals were rotated, recrystallized, or both during shear deformation. We suggest that for crystals with b parallel to the lineation, deformation induced regular layer displacements. For crystals with a parallel to the lineation, periodic displacements of OH planes may have promoted development of regular octahedral cation sequences.

Complex polytypism: Relationships between serpentine structural characteristics and deformation