Saturday, August 5, 2017

"Desert Roses"

“Desert roses” are clusters of crystals of baryte or gypsum containing inclusions of sand. “Baryte” is the new official spelling of the mineral barite. “Desert roses” are also called “sand roses,” “rose rock,” “gypsum rose,” and “gypsum rosettes.” The clusters can be spherical shaped, irregular, or columnar shaped. They are commonly red or reddish brown.

These three pictures shown baryte "desert roses," and the largest one shown above is 2 inches in diameter. They feel heavy to the touch because of the element barium.

"Desert roses" form as evaporites when shallow waters (marine lagoons or lakes) rich in sulfates containing the elements barium or calcite precipitate out of solution. 

“Desert roses” are not that common although it seems that every collector I know has one or two. They are found in Kansas, Oklahoma, Arizona, California, Egypt, and a few other places in the world where lake beds have dried up and become evaporites. They can be of any geologic age (e.g., Permian [250 million years old] or Plio-Pleistocene [less than about 3 million years old]).

Some of the most famous and largest (up to 39 inches tall and weighing as much as 1,000 pounds) specimens of “desert roses” are from the Permian-age (about 275 million years old) Garber Sandstone in Oklahoma. In fact, in 1968, “desert roses” became known as the official state rock of Oklahoma.

Like the one shown above (1.5 inches long), not all clusters of barite contain inclusions of sand nor do they necessarily have to form as evaporites. 

Saturday, July 22, 2017

Sodalite and lapis lazuli

Sodalite can be confused with the rarer and more expensive lapis lazuli (shortened or casual version of this word is lapis), which is also blue. This post deals with how to tell them apart.

Sodalite is a mineral. It is named for its sodium content, consists of the elements sodium, aluminum, silicon, oxygen, and chlorine. It belongs to a group of minerals called the feldspathoids, which resemble feldspars but have a different crystalline structure, a much lower silica content (i.e., feldspathoids are never found in rocks congaing primary quartz), and contain sulfur or chlorine. Sodalite is an ornamental gemstone and is commonly used in jewelry or in making bookends, etc. It is best known for its blue color, but it can also be gray, yellow, green, and commonly mottled in color. It commonly has white veining. It rarely has inclusions of pyrite, and it is not opaque (thus light can transmit through its edges).
Bookends made of sodalite. They are 13 cm hight.

Other side of the bookends shown above.
A small piece of sodalite (5 cm maximum dimension) with a polished surface.
Sodalite has poor cleavage, therefore, it is useful for making carvings of animals. This mineral is commonly found as vein fillings in plutonic igneous rocks (such as nepheline syenites). Associated minerals are microcline, albite, calcite, fluorite, and baryte (barite). It is found in Canada (Ontario, Quebec, and British Columbia), as well as in Maine and Arkansas. 

Sodalite is a "poor man's lapis lazuli."

Lapis lazuli is a metamorphic rock. The most obvious and important  component of this rock is the mineral lazurite, a feldspathoid silicate mineral consisting of sodium, calcium, aluminum, silicon, oxygen, chlorine, and sulfur. It is the presence of sulfur that gives lazurite its intense deep blue color. Most lazurite also contains the minerals calcite (white), sodalite (blue) and sparkling pyrite, as well as small amounts of mica, hornblende, etc.  

The gem form of Lapis lazuli has been prized since antiquity for its deep-blue color. This rock has been mined for thousands of years in Afghanistan and Pakistan (note: "lapis" is an Arabic word). It is opaque, thus light does not transmit through its edges. Pyrite is commonly present, but in minor amounts.
A small piece of polished gem-quality lapis lazuli (3 cm maximum
 dimension). Notice the flecks of pyrite.

Flip side of the lapis lazuli shown above. Notice the vein of calcite
with some pyrite veinlets.
Lapis lazuli takes an excellent polish and can be made into jewelry, carvings, mosaics, ornaments, small statues, and vases. 

Sunday, July 9, 2017


Amazonite: A case study in how a geologist thinks

Amazonite is a bright-green variety of the mineral microcline feldspar. Amazonite occurs in quartz-rich granitic rocks, especially coarse-grained granites called pegmatites, like the one shown here.
This sample is probably from the Pikes Peak region in Colorado, where some of the highest quality specimens are found. The name “amazonite” is derived from the Amazon River because early collectors believed (erroneously) they had found amazonite there.

Amazonite (10 cm maximum dimension) in pegmatite granite. Bright green = microcline; grayish and whitish (both can be somewhat transparent = quartz; white = microcline; black = biotite). The underside of this rock is cuneiform graphic granite (see previous post).
This post presents an opportunity to point out the "visual clues" a geologist would use to explain how this rock formed. 

The rock consists of interlocking large crystals of several minerals. The interlocking of the crystals indicates that they formed from magma (molten material), and the large size of the crystals means that they cooled very slowly. The rock, therefore, is a plutonic igneous rock that cooled very slowly underground. The word "plutonic" is derived from the name of the Roman god, Pluto, who lived underground.

The presence of quartz means the rock formed late in the fractional crystallization sequence. As the magma cooled, a certain sequence of  minerals form, and the chemistry of the remaining melt changes.
This sequence is elegantly summarized by what is known as the Bowen Reaction Series (see diagram at the end of this post).

The presence of lamellae of different colors (green and white) in the overall bright green crystals means that there was exsolution of two minerals: white is albite, and green is microcline. These two minerals crystallized together when the remaining magma melt was rich in potassium, with a lesser amount of sodium. These lamellae form what is known as perthitic texture, which is common to the alkali feldspars (late-forming minerals rich in potassium). 

Amazonite (3.8 cm thick), showing exsolution lamellae of albite (white color).

 The bright green color of amazonite was a mystery to science until detailed studies showed that its color is a result of natural radiation of microcline containing a relatively high level of lead and water in the crystalline structure.  

This poster depicts a poster I made that shows the progressive sequence of fractional crystallization of the Bowen's Reaction Series. It was not made with the intention of showing it online. This explains why the the writing on the poster is somewhat hard to read. Although the dark minerals do not show up well, the poster conveys the concept of the sequence of minerals that form in  an ideal (in a chemical composition sense) magma as it cools. 

Monday, June 26, 2017

Tourmaline-bearing granite

A granite is very distinctive looking if it contains "clusters, spots, clots, or patches" of jet-black tourmaline crystals surrounded by white feldspar crystals.  Such a white-colored granite is   leucocratic (i.e., dark-colored minerals absent or, in this case, concentrated).  Black tourmaline is called schorl, and it is black because of its iron content.  Tourmaline, which is a boron-silicate mineral, is commonly found in pegmatites.  In my previous post, I discussed that pegmatites are associated with the late stages of the cooling history of granite-producing magmas.

A single large crystal (4.75 cm tall = 1.87 in.) of schorl is shown in the following image.  The overall shape of the crystal is triangular  and has striations on all of its sides. The provenance of this crystal is unknown.

Three small boulders (all about 13 inches maximum length) of tourmaline granite are shown below.  A 3/4 of an inch in diameter penny (United States) is used for scale. The "clusters" of tourmaline can be as large as 8 cm across.  The provenance (original location) of this granite is not known to me, but the boulders occurred as rock debris emanating from a man-made dam built in a stream bed in northern Los Angeles County.  I was not sure about the identification of the black mineral in these rocks, so I asked my friend and colleague, Dr. Larry Collins, who is a professor emeritus of geology to take a look at the mineral. He is an expert in mineralogy and petrology, and he recognized the mineral as tourmaline.

In this image, the tourmaline crystals are more spread out, with feldspar and quartz in between. 

This image is a closeup showing a divergent fibrous aggregate of acicular (needle-like) tourmaline crystals, which are concentrated in the upper half of the image. The tourmaline in the lower half of the image is blocky. The entire field of view is about 1 cm in height.

The image below shows a small of piece of a tourmaline-bearing granite (5.5 cm width) from a pegmatite at the Stewart Mine near Pala, San Diego County, Southern California. These crystals of schorl are somewhat massive (structureless).

Tuesday, June 13, 2017

Graphic granite

Graphic granite is relatively common rock consisting of alkali feldspar (i.e., rich in potassium, in some cases in combination with sodium) and quartz, but the rock has a very interesting texture, consisting of a distinctive repetitive pattern that resembles cuneiform writing.

The above picture and the following two pictures are of the same piece of graphic granite, which is about 7 inches long (= 18 cm; the scale is in centimeters).

The origin of graphic granite was debated for over a century. It is now known to be the result of simultaneous growth of quartz (gray color in the rock above) and feldspar (white color) under conditions that favor the planar growth of the feldspar host. 

The next two pictures are different views of the same piece of rock, but you can notice how the texture differs, depending on the view.

Graphic granite occurs in pegmatites, which form during the final stage of a magma's crystallization. The graphic granite illustrated in these three pictures came from the pegmatite at the Stewart Mine in San Diego County, Southern California (see my archived post for  September 30, 2016 which focuses on the mineral rubellite from the Stewart Mine).

Monday, May 29, 2017

Polka-dot granite

"Polka-dot granite" is a distinctive rock, which has been found at several localities in Southern California. It has been used to indicate offset along the San Andreas Fault system, but some geologists have reported that this distinctive granite might not all have been derived from the same magmatic source. More geochemical research is needed.

“Polka-dot granite” is a granite with clots of cordierite (a mineral containing magnesium), biotite, garnet, and other minerals surrounded by an irregular halo (absorption sphere) consisting of white granite with little or no mafic (dark) minerals. The inner and outer margins of the halo are irregular but distinct. The halo is surrounded by the same granite that occurs in the center of the halo. The result looks like large polka-dots, which range in diameter from less than a centimeter to 8 cm.

"Polka-dot granite" collected from Southern California by Dave Liggett.

Same specimen shown above but photographed in the shade.
"Polka dot" is 4.4 cm in diameter.

Close-up of previous photograph.

Another "polka dot" (1.5 cm diameter) from the same locality.

By the way, cordierite is known for its ability to withstand extreme temperatures. It is used for making "pizza stones" because you can take the stone from a low temperature to a very high temperature without the risk of breaking it.

Tuesday, May 16, 2017

Crystals That Show Twinning

In my March 31, 2017 post concerning a distinctive granite, I showed an orthoclase crystal with twinning, which occurs when two separate crystals of the same substance share some of the same crystal lattice. Instead of a normal single crystal, the crystalline structure appears doubled. 

In this new post, I show some other common examples from my personal collection of twinned crystals. They are aragonite, pyrite, gypsum, quartz, and staurolite. 

aragonite (40 mm length)
gypsum (37 mm length)

pyrite cubes (47 mm length)
quartz (47 mm length)
staurolite (18 mm length)
pseudomorph of staurolite (40 mm)

pseudomorph of staurolite (40 mm length)

pseudomorphs are formed
when a mineral is replaced
by a foreign substance