Rock Picture of the Week Archive 5

Spinel

Photo by Jim Bryan

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

Spinel (MgAl2O4) (although iron and chromium often are included) is a fine gemstone that appears in many colors. The red is very similar to, and sometimes substituted for, Ruby. Some historically famous "Rubies", such as the Black Prince's Ruby, were actually Spinel.

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Okenite

Photo by Jim Bryan

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

Okenite (CaSi2O5-2H2O) often forms radiating clusters resembling cotton balls. It is a secondary mineral resulting from the alteration of basalt and can sometimes be found in basaltic geodes.
Be wary of purchasing colored Okenite as there are dyed specimens on the market.

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Cervantite

Photo by Jim Bryan

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

Cervantite (Sb2O4) is formed from the alteration of the mineral Stibnite, and is a secondary ore of the metal antimony.
This specimen is a pseudomorph of Antimony, meaning that it retained the crystal structure of Antimony despite having it's own crystal habit.

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Clintonite

Photo by Jim Bryan

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

Clintonite (Ca(Mg,Al)3(Al3Si)O10(OH)2) belongs to the mica group. It appears in many colors, including colorless, yellow, green, brown, and red, and is more brittle than most micas.
It is often found in the contact areas of metamorphic limestones.

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Calcite

Photo by Jim Bryan

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

Calcite (CaCO3) is very common in sedimentary rocks and may appear in igneous and metamorphic rocks as well. It may be tinted with many different colors, although generally is clear or white and will dissolve in most acids.
Limestone, sea shells, and stalactites are mainly composed of Calcite,

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Apatite

Photo by Jim Bryan

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

Apatite (Ca5(PO4)3(Cl,F,OH)) is a group of minerals consisting mainly of Chlorapatite, Fluorapatite, and Hydroxylapatite, that grade into each other.
The crystals are usually green, but can appear in many other colors. Although fairly soft, it is sometimes used as a gemstone.

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Stilbite

Photo by Jim Bryan

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

Stilbite (NaCa2Al5Si13O36-14H2O) is a fairly common zeolite mineral that is popular among collectors because it has a crystal structure that often takes the shape of a sheaf of wheat, or a semi-hourglass shape.
It is too soft to be used as jewelry and has no significant commercial value.

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Azurite

Photo by Jim Bryan

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

Azurite (Cu3(CO3)2(OH)2) is a deep blue mineral found in the upper, oxidized, portions of copper veins. It is often associated with, and eventually degrades into, the mineral Malachite.
It is sometimes used in ornamental applications, but softness and lack of stability limit its usefulness.

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Quartz

Photo by Jim Bryan

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

Quartz (SiO2) is one of the most common minerals on earth. It appears in many different colors and is often used as a semi-precious gemstone. In jewelry it is sometimes used as a substitute for more precious gemstones.
Some popular varieties are Amethyst, Smokey, and Rose Quartz.

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Fluorite

Photo by Jim Bryan

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

Fluorite (CaF2) is one of mineral collector's favorites because of its often large, colorful crystals that also glow in UV light.
This specimen, with its blue-violet color, shows the rare botryoidal form.

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Opal

Photo by Jim Bryan

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

Opal (SiO2+nH2O) is commonly found in igneous and sedimentary rock cavities. It contains various amounts of water which affects its stability.
Fire Opal is a precious gemstone variety often used in jewelry and ornamentation. This specimen is from Virgin Valley, Nevada.

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Rhodochrosite

Photo by Jim Bryan

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

Rhodochrosite (MnCO3) is a minor ore of manganese. Its attractive coloration often overcomes its softness when used in jewelry or decorations, usually as carved objects.
Rhodochrosite is the state mineral of Colorado.

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Barnacles

Photo by Jim Bryan

Barnacles (Balanus), often called acorn barnacles, have been around for millions of years. These are about 4 million years old.
They excrete a powerful glue and bond themselves to a firm surface on or near the oceanic water line. Inside the shell is a shrimp-like animal that sifts the passing water for plankton. When exposed to the air at low tide, it seals itself up inside.

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Colemanite

Photo by Jim Bryan

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

Colemanite (CaB3O4(OH)3-H2O) is found in evaporative ponds and is the result of decomposition of other boron minerals, such as Borax and Ulexite.
It is mined as a source of boron and used in cleaning agents.

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Acanthite

Photo by Jim Bryan

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

Acanthite (Ag2S) is a low temperature version of silver sulphide. Crystals are rare and most of them found are actually pseudomorphs of higher temperature, but unstable, Argentite. This specimen also has Quartz crystals in the mix.
The dark tarnish on silver utensils is Acanthite.

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Garnet

Photo by Jim Bryan

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

This Garnet specimen is a cross between the Spessartine and Almandine varieties. (Fe3,Mn3)Al2(SiO4)3). Other varieties are Pyrope, Andradite, Grossular, and Uvarovite.
They have been used for centuries as gemstones and as an abrasive substance. Although Garnets are quite common, gemstone quality stones are rare.

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Cerussite

Photo by Jim Bryan

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

Cerussite (PbCO3), sometimes called white lead, is a major ore of the metal lead.
White lead was used in paint for many years until toxicity issues made it illegal to use.

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Vivianite

Photo by Jim Bryan

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

Vivianite (Fe3(PO4)2-8(H2O)), this specimen on sandstone, is generally found in sedimentary rocks, often in or around fossils.
Large crystals are rare, this one came from Bolivia.

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Red Coral

Photo by Jim Bryan

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

This is fossilized Red Coral from Moab, Utah. The end is polished to show the color and pattern.
Coral is an animal that forms a calcium structure. There are many different types that form a wide variety of shapes and colors. They have been around for hundreds of millions of years.

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