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.