Authigenic Components

Calcite - CaCO3
F/S: Anhedral, micritic aggregates, to subhedral -blocky polyhedral. Gz: Authigenic calcite from pelagic rain may be quite uniform blocky polyhedra in a narrow size range (typically  8-30µm),  or else a fine uniform micrite (< 5 µm).   With increasing magnesium, more prismatic forms appear.   R: Variable,  Mod. high RI = 1.486-1.658, lower than dolomite,  weak twinkling; Bf: extreme bf 0. 172,  pastel shades of yellow to green; XS: trigonal; C: perfect cleavage but rare in authigenic crystals; E: extinction parallel to elongation and cleavage. D: Extreme birefringence and relief.


Aragonite- CaCO3
F/S: Acicular needles to rice/wheat-like, thin prisms or ellipsoids. Gz: ca. 10µu. T/C: colorless and transparent. R:  Mod. RI= 1.530-1.686, varies w/ direction.  Bf:  Extreme (0.156) high order pastel yellows, greys, white.  A: May alter to calcite or siderite. XS: Orthorhombic C: Distinct parallel to length of crystals.  P: Absent  E<: Straight Tw: Contact and penetration twins on {110}  OC: Biaxial negative O: Aragonite in lakes almost always a surface water precipitate.  Aragonite generally represents a lake of higher concentration and Mg/Ca molar ratios   5-12.  D:  Distinguished from calcite by form.

F/S: Gz: ca. 10µu. T/C: R: Bf: A: XS: C: P: E<: Tw: OC: O: D:

Siderite - FeCO3
F/S:  R:  Bf:  XS:  C:  E:  D:

Manganese Carbonates

Pisoliths, onchoids, oncholiths

Sulfide Minerals (Pyrite, Greigite, Mackinawite, pyrhottite)

Fe/Mn Nodules




Clay Minerals

Zeolites  Hydrous Aluminosilicates.  Zeolite are a group of about 33 natural minerals formed from the alteration of volcanic ash or other pyroclastic material.  Only three- phillipsite, clinoptilotite, and analcime, are common.  Pictured is Phillipsite.
F/S:  Elongated euhedral to subhedral prismatic crystals, discrete to cruciform twinning.  Gz: 8-250 µu. T/C: Transparent and colorless to yellowish.  R: Mod. to High, 1.481 to 1.486  Bf: Weak, first order grays.  A: Usually clear and unaltered.  XS: Monoclinic C:   P: Pleiochroism absent E<: Straight Tw: Sector, oftern complex.  Simple cruciform. O:   Rare.  Occurs in altered tephras. D:  Elongate prismatic crystals, twinning, high relief, low birefringence.


Silica: Opal, Chert