CHALCEDONY

Chalcedony – SiO2 – is a common tectosilicate, a generic term given to all varieties of quartz popularly known as agate, jasper, chert, chrysoprase, onyx, pietersite and others.

Scientifically, it designates clusters of aligned quartz microcrystals that form a fiber-like structure. These “fibers” can be of three types, formed by quartz crystals aligned parallel or perpendicularly or obliquely in relation to their “z” axes. Chalcedony occurs in many different geological environments, has a number of uses and has (as agate, jasper, chert, etc.) considerable economic importance. Agate is historically a material of high commercial value.

There are more than 70 varieties of agate, 16 varieties of jasper and 3 varieties of chert, depending on the structures formed, the places of occurrence and the colors they present.

1. Characteristics

Crystal system: Trigonal, trapezohedral.

Color: Colorless, white, gray, it can be red, black or any other color. Multicolored is common

Habit: Massive, botryoidal, mammelon, banded.

Cleavage: No.

Tenacity: Brittle.

Twinning: No.

Fracture: Conchoidal.

Mohs Hardness: 6.5 – 7

Parting: No.

Streak: White

Lustre: Silky, resinous.

Diaphaneity: Transparent.

Density (g/cm³): 2.6

 

2. Geology and Deposits

Chalcedony is very common. It is a typical filling of open spaces, which can be fractures in any type of rock, gas bubbles (vesicles) in volcanic rocks (very common!) or dissolution cavities in carbonate rocks. It is the main constituent of fossil woods, where it filled plant cells. It occurs as a filling of cavities in fossils, both in vertebrates and in invertebrates. May form crusts in low and medium temperature hydrothermal veins.

It is a major constituent in silica-rich marine rocks such as diatoms. Forms nodules (“flint”) and levels in limestone rocks. It also forms the cement of some detrital sedimentary rocks such as sandstones (silicified sandstones). It fully replaces other minerals, forming pseudomorphoses.

 

3. Mineral Associations

It occurs in so many geological environments that a list of associated minerals will always be incomplete.

As it often occurs in vesicles (gas bubbles) of volcanic rocks, it is associated there with other secondary minerals typical of this situation: clay minerals (celadonite), macrocrystalline quartz (rock crystal and amethyst varieties), calcite, fluorite, zeolites (many different), fluorapophyllite, gypsum, opal and many others.

In other situations, it occurs with goethite, hematite, pyrite, siderite, etc.

 

4. Transmitted Light Microscopy

Refraction indices:  ne: 1.538 – 1.543      no: 1.530 – 1.533

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless, sometimes pale brown.

Never shows pleochroism.

Relief: Low positive or low negative.

Cleavage: No.

Habits: Chalcedony is composed of quartz crystals with submicroscopic sizes, which can be lined up with their z axes parallel to the base (“lying down”), with their z axes perpendicular to the base (“standing”) or with their z axes oblique to the alignment. This arrangement gives rise to a structure that resembles fibers.

The “fibers” always tend to form spherical aggregates of radial structure. When these radial aggregates interfere with neighboring aggregates, form straight lines at the contacts.

Many aggregates that have grown parallel to each other create a layer-like structure, the typical banding of agate, for example.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Low birefringence, between 0.005 to 0.009, with interference colors in shades of gray to white.

Extinction: Parallel. Spherical aggregates of radially arranged chalcedony fibers show extinction similar to a uniaxial cross in Convergent Light.

Elongation sign: It can be ES(+) or ES(-), depending on the orientation of the quartz grains that are stacked. Chalcedony has ES(-), quartzine has ES(+).

Twins: No.

Zoning: No.

CONVERGENT LIGHT

Character: U(+), very difficult to verify, almost impossible. Could be anomalous biaxial.

2V angle: Often presents a 2V angle of up to 20°, but this is difficult to observe.

Alterations: does not alters, like quartz.

May be confused with: its habit is very characteristic. Crocidolite, the fibrous form (asbestos) of riebeckite (sodium amphibole), is pleochroic in blue, green, and yellow tones.

 

When chalcedony growth nuclei not interfere with other ones, they develop perfect spherical shapes. In XPL, as here, the straight extinction of the “fibers” resembles a uniaxial cross of interference figures.
Multiple chalcedony growth nuclei side by side, barring each other's growth and creating the interference surfaces (straight contacts). XPL.
Chalcedony 01: in PPL, it is colorless with low relief. Brown substance is volcanic glass (obsidian). Rectangular white crystals are quartz.
Chalcedony 01: The same section as in the previous figure, now in XPL. Chalcedny shows the typical "fibrous" pattern, obsidian in isotropic.
(A) When the growth of chalcedony starts with lots of crystallization nuclei side by side, it produces a “layer” with the “fibers” arranged in parallel. (B) When the number of nuclei is smaller, partial spherulites tend to form. XPL
Fracture in limestone rock filled with chalcedony and quartz. Chalcedony starts growing on both sides of the fracture. Sometimes the two growth fronts meet (right side of the image) and sometimes remain a space between the fronts, in this case filled with quartz grains (left side of the image). XPL.
Vesicle (gas bubble) in basaltic rock (black at bottom of the image) filled with quartz crystals (gray/white) and chalcedony – both between the quartz and in the center of the vesicle. XPL.
Quartz crystal in a vesicle in basaltic rock with chalcedony spherulites growing on its faces. XPL.
In a sandstone, grain of chalcedony cemented to neighboring grains with latter silica cement. The grain is formed of numerous small partial spherulites, each one with a "extinction cross".
Immature sandstone with angular grains of orthoclase, microcline, quartz, plagioclase, muscovite, etc. and a grain of chalcedony (center of image) with portions of chalcedony spherulites.
In XPL, in a basaltic rock, vesicle (gas bubble) initially filled with chalcedony at the edges, in the form of isolated spherulites or side by side. The posterior filling was calcite (cream, twinned).
In XPL, basaltic rock (very dark, below) and, above it, minerals that filled a vesicle. Quartz forms idiomorphic crystals (grey/white) and chalcedony forms spherulites on the faces of the quartz crystals.
Chalcedony 02: In basaltic rock, vesicle (gas bubble) filled with chalcedony and a fibrous zeolite. Chalcedony has a yellowish color, forming a first layer of the filling and spherulites in the middle of the filling. It is probably opal recrystallized to chalcedony, which is the common origin of chalcedony. PPL.
Chalcedony 02: same situation of the previous image, but now in XPL. The spherulites show a black cross because the extinction of the “fibers” is parallel and because they are arranged radially.
Chalcedony 02: Same vesicle filling as in the two previous images, but now a detail of it. Basaltic rock at the bottom, fibrous zeolite is white/gray.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of chalcedony. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with chalcedony.

Sample preparation: Chalcedony is easy to polish and ends up polishing better than macrocrystalline quartz and other minerals of similar hardness.

PLANE POLARIZED LIGHT – PPL

Reflection color: Very dark gray.

Pleochroism: No.

Reflectivity: Very low (4%?)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Anisotropy was not observed.

Internal reflections: Generalized in very light colors in the colorless chalcedony, changing according to the macro color of the chalcedony, which makes evident any parallel bands that may exist.

May be confused with: many other light colored transparent minerals with no obvious diagnostic features. Banding is diagnostic.

Chalcedony 01: in PPL, chalcedony is gray and a bit of banding can be seen.
Chalcedony 01: in XPL the banding typical for many chalcedonies can clearly be seen, the so-called "agate banding".
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