Rock Picture of the Week Archive 4

Photo by Jim Bryan

This specimen is on display at the California State Mining and Mineral Museum in Mariposa, CA.

Copper (Cu) is usually found chemically bonded with other elements, forming minerals such as Malachite, but sometimes in its native state.
It is the first metal to generally be used in tool making. Today it is used extensively in wire due to its high electrical conductivity and availability.

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Benitoite (2)

Photo by Jim Bryan

This specimen is on display at the California State Mining and Mineral Museum in Mariposa, CA.

Benitoite (BaTiSi3O9) revisited. Discovered in 1907, this beautiful blue gemstone is one of the rarest in the world. The only place it was mined commercially is in San Benito County, CA, and is the state gemstone. The known deposits are now mostly depleted. Its blue color probably comes from titanium. It does come in a few other colors, although they are not as common.

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Liroconite

Photo by Jim Bryan

This specimen is on display at the California State Mining and Mineral Museum in Mariposa, CA.

Liroconite (Cu2Al(AsO4)(OH)4+4(H2O)) is a rare mineral sometimes found in the oxidized portions of copper veins.
Although very colorful, it is soft enough to be scratched by your finger nail.

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Hematite and Limonite

Photo by Jim Bryan

This specimen is on display at the California State Mining and Mineral Museum in Mariposa, CA.

Hematite (Fe2O3) with a partial coating of Limonite. Hematite is a major iron ore and Limonite is a generic name often attached to various hydrated ores of iron.
The yellowish rust created when water contacts iron is an example of Limonite.

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Andradite

Photo by Jim Bryan

This specimen is on display at the California State Mining and Mineral Museum in Mariposa, CA.

Andradite (Ca3Fe+++2(SiO4)3) is a calcium-iron Garnet. Although it comes in many colors, popular varieties are Dementoid (green), Melanite (black), and Topazolite (yellow).
It has the proper characteristics to make gemstones, but gem quality stones are rare and fairly valuable.

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Crinoid

Photo by Sharon Munford

Crinoids, sometimes called sea lilies, have three major parts. The stem is segmented and attaches to the bottom, the body or calyx (partial imprint shown here) contains the organs, and arms that gather food.
Some varieties have survived the mass extinctions, but are not as plentiful as they were when this one was alive, about 330 million years ago. These fossils are common in the limestones of central USA and other areas.

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Descloizite

Photo by Jim Bryan

This specimen is on display at the California State Mining and Mineral Museum in Mariposa, CA.

Descloizite (Pb(Zn,Cu)(VO4(OH)) is a rare secondary mineral found in the oxidized portions of metal deposits, and is a minor ore of lead, zinc and vanadium.
It contains more vanadium than copper, and as copper replaces vanadium it grades into the mineral Mattramite.

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Aragonite

Photo by Jim Bryan

This specimen is on display at the California State Mining and Mineral Museum in Mariposa, CA.

Aragonite (CaCO3), along with Calcite, are polymorphs of calcium carbonate. This means that they both have the same chemical formula, but crystalize differently.
Aragonite is less common than Calcite and usually forms in lower temperature and pressure conditions.

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Vanadinite

Photo by Jim Bryan

From the private collection of Mike & Chris Whittier of Rocks in a Hard Place.

Vanadinite (Pb5(VO4)3Cl) is sometimes found in the oxidized zones of lead mines. It is one of the major sources of the metal vanadium and a minor ore of lead.
These attractive crystals make great specimens, but are too soft to be used as gemstones.

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Celestite

Photo by Jim Bryan
From the private collection of Mike & Chris Whittier of Rocks in a Hard Place.

Celestite (SrSO4) is named for it's usual sky-blue color, but can appear in many other colors including clear, yellow, green, red, or brown. Crystals can be very large, sometimes weighing many pounds.
It is too soft to be used as a gemstone, but is an important source of strontium compounds.

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Knightia

Photo by Jim Bryan

Knightia is an extinct type of herring that lived about 50 million years ago and is the state fish fossil of Wyoming. This one came from the Green River formation where they are very common. There are places where you can dig and keep these and similar specimens.

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Apophylite

Photo by Jim Bryan

From the private collection of Mike & Chris Whittier of Rocks in a Hard Place.

Apophylite ((K,Na)Ca4Si8O20(F,OH)-8(H2O)), originally a mineral name, is now just the group name for Fluorapophylite, Hydroxyapophylite, and Natroapophylite. This specimen is most likely Fluorapophylite.
The name comes from the way it flakes off when heated.
The Apophylites are fairly common and and are used mainly as specimens.

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Phlogopite

Photo by Jim Bryan

From the private collection of Mike & Chris Whittier of Rocks in a Hard Place.

Phlogopite (KMg3(Si3Al)O10(F,OH)2) is sometimes known as magnesium mica. The color can be yellow, greenish, or red-brown (as in this specimen).
It has many of the physical characteristics of Biotite and, as the iron content increases, grades into it.

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Tanzanite

Photo by Jim Bryan

From the private collection of Mike & Chris Whittier of Rocks in a Hard Place.

Tanzanite (Ca2Al3(SiO4)(Si2O7)O(OH)) Is a bluish variety of Zoisite. It is a rare gemstone found only in Tanzania and the government controls its release.
Almost all of the gems are heat treated to intensify their color.

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Cinnabar

Photo by Jim Bryan

From the private collection of Mike & Chris Whittier of Rocks in a Hard Place.

Cinnabar (HgS) is the primary ore of the metal mercury. It most commonly appears as small crystals and masses. Large crystals like this one are rare.
It has historically been used as a red coloring agent, but is very toxic. Today, one of the main uses is in fluorescent lights.

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Labradorite

Photo by Jim Bryan

From the private collection of Mike & Chris Whittier of Rocks in a Hard Place.

Labradorite ((Ca,Na)(Al,Si)4O8) is a member of the feldspar group of minerals. It was first found on the Labrador peninsula, Canada.
It is sought after for it's iridescence and is often used in decorations and jewelry.

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Gypsum Rose

Photo by Jim Bryan

From the private collection of Mike & Chris Whittier of Rocks in a Hard Place.

Gypsum roses (CaSO4-2H2O), sometimes called Selenite roses, are flower-like crystals often formed from the evaporation of mineralized lakes and seas. Gypsum crystals can form rapidly (geologically speaking). Some over 20 feet long, from a mine in Mexico, formed in less than 500K years.
Gypsum is the main component of sheetrock in houses.

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Christobalite

Photo by Jim Bryan

From the private collection of Mike & Chris Whittier of Rocks in a Hard Place.

Christobalite (SiO2) is a high-temperature silicate. It is a polymorph of Quartz, which means that it is chemically the same with a different crystal structure. It is formed when igneous rock, in this case obsidian, cools slowly enough to allow crystallization.
It usually occurs in globular masses with small crystals. Large crystals are rare.

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Rutile

Photo by Jim Bryan

From the private collection of Mike & Chris Whittier of Rocks in a Hard Place.

Rutile (TiO2) is formed in high temperature and pressure environments. It is often included in, or growing out of, other crystals. Cat's eye and star gems are the result of small, hair-like, inclusions of Rutile crystals .
It is a major ore of titanium, used as a pigment, and is an ingredient in some sunscreens.

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