Coccolith crystals of Pleurochrysis carterae: crystallographic faces, organization, and development

The crystallographic and morphological configuration of the mineral ring as sociated with the coccoliths of Pleurochrysis carterae was determined by tr ansmission electron microscopy and electron diffraction. Mature Pleurochrys is coccoliths consist of an oval organic base plate, a distal rim of interl ocking calcite crystals, and a narrow ribbon of organic material which teth ers the mineral ring to the base plate. Crystals of two distinct forms (R a nd V units) alternate about the rim in a quasi regular manner; their crysta llographic c-axes are aligned parallel to and inclined about 63 degrees to the coccolith plane, respectively. The mineral ring has four platelike elem ents: the distal-shield and outer-tube elements which form the V unit, and the proximal-shield and inner-tube elements which form the R units. The pla ty surfaces of both tube elements correspond to the common (10 (1) over bar 4) rhombohedral faces of calcite, and the plates of the proximal-shield el ement are prismatic (2<(11)over bar>0) faces. The plates of the distal-shie ld element are rather curved and their orientation does not correspond to a favorable calcite face; however, for convenience they are described as app roximately ((1) over bar 108) faces, faces which rarely, if ever, develop i n inorganic sources of calcite. During coccolith development the earliest h abits observed for both V and R units correspond to rectangular parallelepi peds. Outgrowth from the initial V unit begins by expansion of (10 (1) over bar 4) faces which form the platy surfaces of the outer-tube element. Thro ughout this period of development the mineral ring is flexible, at least in an isolated state. Subsequent outgrowth of the inner-tube and proximal-shi eld elements from the initial R unit produces a rigid interlocking ring. Th e unusual ((1) over bar 108) faces of the distal-shield element develop aft er the crystals are locked in place. Organic structures in intimate associa tion with the mineral phase during its nucleation and growth include the co ccolith ribbon, the calcium-polyanion particles, and the membrane of the co ccolith vesicle. These structures are described in reference to their putat ive functions in regulating the development of V and R units.