Geology Reference
In-Depth Information
Acicular:
Needle-like crystals, growing normal to the substrate. Crystals elongated parallel to
the c-axis, exhibiting straight extinction. Terminations are pointed or chisel-shaped, twinning is
common. Width < 10
m, length about 100
m and more. Often forming isopachous crusts.
Predominantly aragonite, but also Mg-calcite. Marine phreatic. Pl. 31/2, Pl. 34/1.
Fibrous:
Fibrous crystals, growing normal to the substrate. Crystals show a significant length
elongation, usually parallel to the c-axis. Crystal shape is needle-like or columnar (length to width
ratio > 6:1, width > 10
m). Size commonly fine to medium crystalline. Often forming isopachous
crusts; common in inter- and intraparticle pores. Aragonite or High-Mg calcite. Mostly marine-
phreatic, but also meteoric-vadose and marine-vadose (columnar crystal shape). Syn.: Radial
fibrous. Pl. 2/4, Pl. 31/1-2, Pl. 32/1-4, Pl. 50/6.
Botryoidal:
Pore-filling cement made of individual and coalescent mamelons exhibiting discon-
tinuous horizons, e.g. dust lines ranging in size from tens of microns to several centimeters. The
cement consists of individual and compound fans, which in turn are composed of elongated
euhedral fibers with a characteristic sweeping extinction in cross-polarized light. Aragonite. Usu-
ally marine (common in cavities of reefs and steep seaward slopes), but also known from burial
environments. Syn.: Spherulitic. Pl. 145/1-3.
Radiaxial fibrous:
Large, often cloudy and turbid, inclusion-rich calcite crystals with undulose
extinction. Size medium to coarse crystalline. Sometimes extending several millimeters in length,
usually about 30 to 300
m. Crystal length/width ratio 1:3 to 1:10. Crystals show a pattern of
subcrystal units. Within each subcrystal that diverges away from the substrate an opposing pat-
tern of distally-convergent optic axes occurs, caused by a curvature of cleavage and twin lamel-
lae. Undulose extinction of subcrystals or subcrystal units are used in distinguishing three radi-
axial subtypes (see text). Often forming isopachous crusts. Phreatic-marine and burial. Pl. 27/2,
Pl. 34/2; Fig. 7.9.
Dog tooth:
Sharply pointed acute calcite crystals of elongated scalenohedral or rhombohedral
form, growing normal and subnormal to the substrate (grain surfaces, atop earlier cements).
Crystals are a few tens to a few hundred micrometers long and have acute and sometimes
blunted terminations. Often meteoric and shallow-burial but also marine-phreatic and hydrother-
mal. Syn.: Bladed scalenohedral cement, bladed prismatic calcite cement, dentate cement,
scalenohedral calcite cement. Pl. 2/3, Pl. 31/5-6, Pl. 34/8.
Bladed:
Crystals that are not equidimensional and not fibrous. They correspond to elongate
crystals somewhat wider than fibrous crystals (length/width ratio between 1.5:1 to 6:1) and ex-
hibiting broad flattened and pyramid-like terminations. Crystal size up to 10
m in width and
between less than 20 and more than 100
m in length. Crystals increase in width along their
length. Commonly forming thin isopachous fringes on grains. Usually High-Mg calcite but also
aragonite. Marine-phreatic (abundant in shallow-marine settings) and marine-vadose. Pl. 33/1,
3, 5, 8; Pl, 34/7.
Dripstone:
Pendant cement characterized by distinct thickening of cement crusts beneath grains
or under the roofs of intergranular and solution voids. The cement forms on droplets beneath
grains after the bulk of the mobile water has drained out of the pores, leaving a thicker water film
at the lower surface of the grains. Forms typically gravitational, beard-like patterns. Predomi-
nantly calcite. Formed below the zone of capillarity and above the water table within the mete-
oric-vadose zone (often associated with meniscus cement), but also in the meteoric-phreatic and
sporadically in marine-vadose diagenetic environments (e.g. inter- and supratidal, and beach-
rocks: aragonitic dripstone cement). Syn.: Gravitational cement, microstalactitic cement, micro-
stalactitic druse cement, stalactitic cement. Pl. 34/6, Pl. 126/1.
Meniscus:
Calcite cement precipitated in meniscus style at or near grain-to-grain contacts in
pores containing both air and water. Exhibits a curved surface below grains. Resulting inter-
granular pores have a rounded appearance due to the meniscus effect. Characteristically formed
in the meteoric-vadose zone but may also occur in the phreatic-meteoric and the vadose-marine
environment (beachrock). Pl. 14/1, Pl. 32/5-6, Pl. 33/4, Pl. 126/1.
Drusy:
Void-filling and pore-lining cement in intergranular and intraskeletal pores, molds and
fractures, characterized by equant to elongated, anhedral to subhedral non-ferroan calcite crys-
tals. Size usually >10
m. Size increases toward the center of the void. Displays a characteristic
fabric (see Fig. 7.12). Near-surface meteoric as well as burial environments. Syn.: Drusy calcite
spar mosaic, drusy equant calcite mosaic. Pl. 10/2.
Fig. 7.8.
Cement types. Part 1.
