Varieties of Quartz: Part 1

Quartz occurs in a great number of varieties that differ in form and color. Quartz can occur as single crystals, crystals in druses, massive aggregates, and dense nodules. Quartz is colorless, if pure, but can assume any color because of inclusions of other minerals or because of the presence of built-in trace elements.

The more common varieties have been given their own names. The different varieties are commonly classified in two major groups:

One of these major groups is called macrocrystalline, which includes varieties that commonly form visible crystals. When people talk about quartz, they mostly think about macrocrystalline quartz. Some examples of macrocrystalline quartz are shown below:

Individual transparent crystals of euhedral quartz (= nice angular crystals), (largest crystal here is 2.3 cm height).

 

A cluster of transparent crystals of quartz (largest crystal 6 cm height). To get crystals like these, they must have free space to grow into.

7 cm height

4.5 cm height

The above two images are of transparent macrocrystalline crystals of quartz containing inclusions of the mineral rutile, thereby forming "rutilated quartz." The acicular (needle-like) crystals of rutile, a titanium oxide, form within the structure of the quartz crystal. The "needles" can be golden, black, or other hues.

Drusy quartz called "cactus quartz"(5.8 cm height) when it coats much larger, singular crystals of quartz.

Amethyst is the purple, violet, or lavender variety of quartz. The colors are the result of natural radiation emanating from the surrounding rock. This radiation affects any iron impurities that might be present in the quartz and consequently causes the coloration. This specimen is 9.5 cm in width.

Citrine is the yellow to orange variety of quartz, with the colors being the result of the presence of iron impurities. This specimen is 8.3 cm height.

Smoky quartz is the gray, brown, or black variety of quartz, with the colors being the result of exposure to radiation.

Rose quartz is the rose or pink variety of quartz. The color is due to nano fiber inclusions (see my May 31, 2016 post). Rose quartz does not form crystals or crystal faces, and it is never transparent. It is, however, classified as a macro crystalline variety of quartz, as it is made up of many inter grown crystals and subindividual crystals. This specimen is polished and 2.3 cm height.

Aventurine (unpolished [4 cm height] and polished pieces) is actually a quartzite (a metamorphic rock, not a mineral) composed on interlocking macro crystalline quartz grains and other color-imparting minerals. Aventure is commonly green but can be orange brown, gray, or blue. The color is due to the presence of small-sized reflective particles of the mineral mica or particles of iron. 

The next post will show some of the common varieties of the second group: cryptocrystalline (or microcrystalline) varieties of quartz.

Cleavage in minerals

Cleavage refers to the specific planes of weakness along which some minerals split. These planes (or directions) are smooth, shiny and parallel to the zones of weak atomic bonding inherent in the structure of the mineral. 

Cleavage is an important “tool” used in the identification of minerals. Unlike crystal faces, which can be also flat and smooth, cleavage is only evident when a mineral is broken.

Some of the more common types of cleavage are shown below:

PINACOIDAL = one direction of cleavage (like a sheet of paper), resulting in thin, flat sheets which can be peeled apart (e.g., mica).

Two views (oblique and side) of a sheet of biotite mica (maximum dimension 26.5 cm). 

RHOMBOHEDRAL = three directions of cleavage with the cleavage planes forming angles other than 90° (e.g., calcite).

rhombohedral calcite, 3 cm high

CUBIC = three directions of cleavage with the cleavage planes forming 90° angles; (e.g., halite).

cubic halite, 7.5 cm high; the lower right side has a piece broken off

OCTAHEDRAL = when a mineral breaks in the form of a diamond, resulting in eight nearly equal faces (octahedron); (e.g., octahedral fluorite).

octahedral fluorite, 3 cm high

DODECAHEDRAL = when a mineral breaks in the form of a polyhedron with 12 faces; (e.g., garnet).

dodecahedral garnet, 4 cm high

Not all minerals have cleavage (e.g., quartz), but all minerals show fracture, which is the tendency of a mineral to break along an irregular surface. In glass and some minerals, like quartz, the broken surface is called conchoidal fracture, and it does not follow any planar surfaces (i.e., crystal faces).

conchoidal fracture in quartz, specimen is 3 cm high

quartz crystal showing natural crystal faces, specimen is  4.5 cm high

Inoceramids

Inoceramid bivalves (clams) range from the Permian to latest Cretaceous. On a global scale, they were widespread and locally abundant during the Jurassic and Cretaceous. They went extinct just before the end of the Cretaceous. Inoceramids lived on the ocean floor and were filter feeders.

This first image is an "average-looking" Inoceramus specimen, showing the characteristic oblique outline of the shell and the prominently raised, concentric ribs. Well-preserved shells have "mother-of-pearl" luster on the inner layer. The outer shell layer consists of well-developed prisms of calcite.

This is a cross-section of a fragment of the prismatic layer of Inoceramus. The fragment is 1 cm in thickness and 2.5 cm in length. Even small pieces of the outer layer of well-preserved Inoceramus shells can show the very characteristic, well-developed (i.e., thick) prismatic shell structure.

Plaster cast (6 cm height) of Inoceramus orientalis ambiguus Nagao and Matsumoto, 1939, Cretaceous (late Santonian), Shasta County, northern California. 

This specimen is from Upper Cretaceous (upper Campanian to lower Maastrichtian) strata in the San Diego area, southern California. The drafting pen is 12.5 cm (5 inches) in length.

Inoceramids are excellent in fossils for "age-dating" marine rocks. This is because their species evolved rapidly, commonly averaging one per 0.2 to 0.5 million years. A normal bivalve species lasts for about 2 million years. Single species of inoceramids can occur in widely scattered parts of the world. About 75% of species and subspecies have intercontinental to cosmopolitan distribution in the Cretaceous, and, in many cases, exceed the precision of geologic age correlation found in co-occurring ammonite species and subspecies or the marine planktonic microfauna! Because of their widespread distribution, however, they are not very useful in defining paleobiogeographic regions.

Some genera, many species, and many subspecies of inoceramids can be frustratingly similar looking because they have so few determinate morphologic characters. Some researchers use complex statistical analyses (e.g., multivariate analyses) to help them define and recognize species and subspecies. Before the advent of computers, these statistical analyses were overwhelming to do by hand. Now, there is software that can help paleontologists, who must first digitize the morphological data.

Some inoceramid specimens can be large and rather thick walled, like this Cretaceous specimen from Kansas.

Photo is from wikipedia.org

Some species of inoceramids grew to giant size, up to 3 m (9.84 feet = 118 inches) in length, making them the largest bivalves of all time. Their giant size was probably an adaptation for living in murky bottom waters; the gills were probably corresponding large and allowed for survival in oxygen-deficient waters. These giants are all very flat valved. The unusual color of this specimen is because of the exposure of the inner shell layer (mother-of-pearl layer).

While alive, the valves of large inoceramids could serve a shelter for schools of small symbiotic fish, which can be preserved inside as impressions. In many cases, the exterior surfaces of the shells of inoceramids are encrusted by numerous small oysters.

What's in a name?

What’s in a name? That question is extremely important in the world of paleontology.

The principle of scientific naming started with the binominal system of nomenclature established by Carl Linnaeus in 1758. For a species to be officially recognized, it must be given a latinized generic name and specific name (e.g., Turritella andersoni Dickerson, 1916). Note that these names must be italicized. The genus name is always capitalized, whereas the species name is not (this rule is internationally recognized although magazine and newspaper articles commonly do not follow it). The genus name can be abbreviated to a single letter (e.g., T. andersoni Dickerson, 1916), whenever the full name has already been mentioned, so that the reader is aware of the genus name.

Carl Linnaeus, also known as Carl von Linné (1707–1778), a Swedish naturalist.

Image modified from Wikipedia (2018).

The whole concept of naming involves taxonomy and systematics, which are basically synonymous. Taxonomy (or systematics) consists of two distinct components: Nomenclature, which deals with the purely legalistic aspect of names, and classification, which deals with the ranking or grouping of various categories of names (e.g., order, superfamily, family, genus, species). In modern times, DNA studies have revolutionized classification schemes.

The rules of nomenclature of genus and species names are given in the “International Code of Zoological Nomenclature” (ICZN). It provides widely detailed information about accepted procedures dealing with how names should be correctly established, which name must be used in case of name conflicts, and how scientific literature must cite names. The first edition of the Code was published in 1961. The present edition (4^th) was published in 2000. A pdf of the Code is available online.

Title page of the ICZN.

If you are interested in the details of nomenclature, Wikipedia.org, has a very informative overview of the principles (e.g., priority, homonymy, type specimens upon which names are based [e.g., the holotype is the "name bearer" of a species], gender agreement of names, etc.). Also, you can Google the phrase “international code of nomenclature.”

Wikipedia also has a good overview of what a synonymy (list of equivalent names) is. Just "Google" the word “synonymy.”

Beale's Cut, Newhall, Southern California

Beale's Cut is a narrow, man-made gap through a ridge near the town of Newhall, in northern Los Angeles County, southern California. This gap (or pass) dates back to 1854, when Phineas Banning dug out a 30-foot gash, in order to allow horse-drawn wagons and stagecoaches to travel through a narrow canyon whose head was blocked by solid rock. The gap occurs in an area that has had a succession of names: Fremont Pass, San Fernando Pass, and more, recently, Newhall Pass.

On the approach to Beale's Cut in April, 1985.
(the view is to the northeast)

Beale's Cut, April, 1985, with a partial silhouette of a person
 and a chain-link fence, for scale.

Beale's Cut, April, 1985.

 Google Earth (2018) image showing a

 bird's eye (vertical) view of Beale's Cut.

In 1861, Edward Fitzgerald Beale deepened the gap to 90 feet in height, thus allowing even better passage to places like Fort Tejon, to the north. At that time, the gap in the canyon became known as Beale's Cut. It was used for vehicles until 1910. It was also used for many silent westerns and was where the American actor Tom Mix and his horse allegedly "jumped the gap," in the movie "Three Jumps Ahead."

Beale's Cut is still in existence, but it suffered during the Northridge Earthquake in 1994. Today, it is only 30-feet deep again because of infilling by rock falls. 

It is located just off of Sierra Highway, which is west of Interstate 14. Spotting if from the road level is difficult. Along the side of Sierra Highway there is a small monument/plaque, but the entire area is fenced off now in order to reduce vandalism and trash dumping. There is no place to turn off the road to park your car. 

The gap occurs in the Saugus Formation, a Pleistocene fluvial (ancient river) sandstone deposit associated with the erosion of the adjacent San Gabriel Mountains.

Miocene spring-deposited tufa in southern California Pt 2

Part 1 concerned an occurrence of Miocene spring-deposited tufa in the Barstow Formation, Calico Mountains, southern California. Part 2 is about another Miocene occurrence of spring-deposited tufa, 118 miles to the southeast of the Barstow locality, in the Diligencia Formation in the Orocopia Mountains, northeast of Salton Sea, Riverside County, California.

Google Earth (2018) image,

The Diligencia Formation spring-tufa deposits resemble fossil "logs," as shown above. These "logs" occur mainly as branching-horizontal tubes, in contrast to the mostly vertical buildups of the Barstow Formation spring-tufa deposits. The Diligencia tufa deposits are confined to a single thin bed that crops out for a least 6 km. This bed represents a lake-shoreline environment, which interfingers with basalt flows and river-delta deposits. The hammer, which is the same one shown in all the subsequent images, is 27 cm in length.

The Diligencia tufa tubes, are commonly parallel to bedding, can be up to 65 cm in length and 25 cm in diameter.  Hammer is 27 cm length.

This is a horizontal tube, which has been extensively weathered.

In some places, the Diligencia tufa deposits are transported accumulations of closely packed tubules concentrated into a thin bed of limestone.

In other places, the Diligencia tufa deposits occur as scattered, weathered remains.

All the Diligencia tufa deposits consist predominantly of wavy microcrystalline calcite laminations surrounding by small core that is filled with darker sediment and/or partly with coarser grained calcite. The "dime" coin is 18 mm in diameter.

Miocene spring-deposited tufa in southern California Pt 1

In 1981-1982, while doing field work on rocks of middle Miocene age in southern California, I and my students came across some interesting tubular and bulbous-shaped geologic features which are sedimentary rock structures, called spring-tufa deposits, made by waters seaping onto the floors of alkaline lakes. Tufa is composed of calcium carbonate (calcite). 

One of the locales is in the Barstow Formation in the Calico Mountains, west of the “ghost town” of Calico, near the town of Barstow, San Bernardino County, Mojave Desert, California.

Google Earth (2018) image

The tufa deposits at the Calico Mtns. locale occur in at least two beds (8 m apart vertically) in a fluvial-deltaic facies. The tufa deposits are laterally persistent but not continuous. About every 15 to 20 m along strike, there are in situ tubes (columns). In between, the tubes are busted apart because of weathering. All the tufa deposits are within the same general area. 

Outcrop of the Barstow Formation, with tufa deposits indicated by the red arrows. There is horizontal development of tubes in the lower right-hand side of photo, scattered horizontal accumulation of tubes on left-side of photo, and a small cluster of fan-shaped tubes on the middle right-hand side of photo. Jacob's staff is 50 cm length; rock hammer is 32 cm length (12.5 in.).

Close-up of same small cluster of fan-shaped tubes shown in the previous photo (hammer is 32 cm length, 12.5 in). Notice the horizontal orientation of the tubes at the base of the cluster, whereas they bend upward with growth. Also, the tube diameter increases upward.

Close-up of some tubes showing corrugated or fluted surfaces. Coin (USA quarter) is 2.5 cm width.

Close-up of some tubes showing their cross section structure (white part of Jacob's staff is 10 cm length). Some of the tubes are hollow, some one-half filled, and others are filled. Tubes vary in diameter, from pencil size to about 10 cm.

Largest cluster, approximately 1.3 m high.

Cross-section of one of the tubes, which shows wavy growth bands. Ruler is in cm (total of 15) on its left side and inches (total of 6) on its right side. The growth bands might have been influenced by microbial and/or algal processes.

Part 2 concerns another Miocene Formation with similar spring-fed tufa deposits. This formation crops out in the Orocopia Mountains, southern California.