Serpentine minerals from Sasaguri area, Fukuoka Prefecture, Japan.

Serpentinized ultramafic rocks and Sangun metamorphic rocks occur in the Sasaguri area, Fukuoka Prefecture. Lizardite and antigorite are the most abundant rock-forming minerals in the Sasaguri ultramafic rocks and are accompanied by lesser amounts of chrysotile (2Mcl and 2Orcl), magnetite, chromite, pyrite, pentlandite, millerite, forsterite, chlorite, talc, tremolite, anthophyllite, calcite, dolomite, magnesite, aragonite, hydromagnesite, artinite, brucite, pyroaurite, coalingite, szaiberyite, sepiolite, and deweylite, Serpentinites can be divided into following four types by the assemblage and grain size of serpentine minerals: (1) lizardite serpentinite, (2) lizardite-antigorite serpentinite, (3) fine-grained antigorite serpentinite, (4) coarse-grained antigorite serpentinite. The representative rock-forming serpentine minerals (five antigorites and one lizardite) and the two vein-forming fibrous chrysotiles in lizardite serpentinite were examined by X-ray, chemical, electron optical, thermal, and infrared absorption analyses. The significant differences in chemical composition and structural properties of antigorties are found. Fine-grained antigorites have low Fe contents (total FeO=1.6-2.5 wt%) and long supercell parameters (A=41.8-42.5 Å), and coarse-grained antigorites have high Fe contents (toal FeO=4.9-6.0 wt%) and short supercell parameters (A=35.4-40.5 Å). Lizardite and chrysotile show low Fe contents (total FeO=1-3 wt%). Many lizardite serpentinites in this area contain chrysotile 2Mcl. Its proportion is approximately 20 to 30 wt%. Povlen-type chrysotile is found in the vein-forming chrysotile specimen by electron microscope examinations.