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, 2H1, 2H2, 6R1, and 6R2 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.
Two-layer (I,II) and three-layer (I,I,II) serpentines and randomly interstratified serpentine-chlorite contain frequent but nonperiodic planar defects perpendicular to a* and the pseudo-a* axes that are interpreted to displace polytypic sequences. These defects predate chloritization and, in some cases, appear to serve as sites for chlorite nucleation. Layer silicates are crosscut by late-stage polygonized two-layer serpentine with disordered or regular stacking. In some cases with regular stacking, enantiomorphic 6R2 segments with a common c-axis direction parallel to their boundary are separated by sectors with 2H2 stacking. Polygonized serpentines are nucleated on steps at the surfaces of layer silicates and may have developed at a late stage by recrystallization of curved serpentine.