Diamond (Prairie Creek Lamproite, Early Cretaceous, 106 Ma; Crater of Diamonds, Pike County, Arkansas, USA) 2
Diamond from Arkansas, USA. (longest dimension of crystal = ~1.65 centimeters; USNM R2-00, National Museum of Natural History, Smithsonian Institution, Washington D.C., USA; public domain photo provided by the USNM)
A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 6100 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.
Elements are fundamental substances of matter - matter that is composed of the same types of atoms. At present, 118 elements are known (four of them are still unnamed). Of these, 98 occur naturally on Earth (hydrogen to californium). Most of these occur in rocks & minerals, although some occur in very small, trace amounts. Only some elements occur in their native elemental state as minerals.
To find a native element in nature, it must be relatively non-reactive and there must be some concentration process. Metallic, semimetallic (metalloid), and nonmetallic elements are known in their native state.
The element carbon occurs principally in its native state as graphite (C) and diamond (C). Graphite is the common & far less valuable polymorph of carbon. A scarce polymorph of carbon is diamond. The physical properties of diamond and graphite couldn’t be more different, considering they have the same chemistry. Diamond has a nonmetallic, adamantine luster, typically occurs in cubic or octahedral (double-pyramid) crystals, or subspherical to irregularly-shaped masses, and is extremely hard (H≡10). Diamonds can be almost any color, but are typically clearish, grayish, or yellowish. Many diamonds are noticeably fluorescent under black light (ultraviolet light), but the color and intensity of fluorescence varies. Some diamonds are phosphorescent - under certain conditions, they glow for a short interval on their own.
Very rarely, diamond is a rock-forming mineral (see diamondite - www.flickr.com/photos/jsjgeology/14618393527).
The rough diamond seen here is from a Cretaceous-aged lamproite body at Crater of Diamonds State Park in Arkansas. At the site, diamondiferous lamproitic rocks of the Prairie Creek Lamproite are exposed (actually, it’s just a field of rocky dirt). Diamonds are found regularly by tourists. The Prairie Creek Lamproite is an ultramafic pipe that intruded Proterozoic-aged (Grenvillian) basement rocks. Prairie Creek rocks are Cretaceous in age and date to about 106 million years old.
A strange myth has emerged over the years that says Arkansas diamonds are harder than diamonds from elsewhere. This is not true. Diamonds have a hardness of 10 on the Mohs Scale, whether they're from Arkansas or not.
This diamond is somewhat rounded due to magmatic resorption.
--------------------------------------------
From park signage:
THE “CRATER” PIPE
The “Crater” diamond deposits are the result of a series of volcanic eruptions that carried a diamond-bearing magma called peridotite to the surface. This molten rock slowly cooled, forming a solid, vertical shaft or pipe.
Later, secondary eruptions occurred that shattered much of the peridotite. The fragmented mixture left by the eruptions formed a volcanic breccia that comprises the majority of the searching area. The intrusive peridotite, and a tuff and fine-grained breccia, also formed by the eruptions, cover the remainder of the mine surface.
STRATIGRAPHIC PROFILE
The volcanic eruptions forming the “Crater’s” pipe had to force their way upward through the layers of rock and sediment formed during Paleozoic and Mesozoic times.
Diamonds are located throughout the pipe - from the surface to an undetermined depth (possibly several thousand meters). The mine is simply the exposed surface area of the pipe.
TOPOGRAPHY
Surface features at the “Crater” today reflect the depositional, erosional, and weathering processes which have occurred since the pipe’s formation. Erosional forces, such as wind and water, have erased the original surface features of the pipe and surrounding rock formations.
Most of the exposed volcanic rocks tend to weather rapidly forming soils of various colors. It is in these soils that we find diamonds today.
In 1948, Glen L. Martin leased property to build a diamond processing plant. Martin’s plant operation lasted only eight months after recovering less than 250 carats of diamonds. The original location of the Martin Plant has been preserved. Its location lies just south of the Visitor Center.
DAZZLING DIAMONDS FROM THE CRATER
Over 85,000 Crater diamonds have been found. Each year an average of 600 are found.
1924 - The Uncle Sam, 40.23 carats
1956 - The Star of Arkansas, 15.33 carats
1956 - The Eisenhower, 6.11 carats
1960 - The Gary Moore, 6.43 carats
Over 24,000 diamonds have been found since Crater of Diamonds State Park was established in 1972.
1975 - The Amarillo Starlight, 163.7 carats
1978 - The Lamle Diamond, 8.61 carats
1981 - The Star of Shreveport, 8.82 carats
1986 - The Conell Diamond, 7.95 carats
1990 - The Strawn-Wagner, 3.03 carats
1997 - The Cooper Diamonds, 6.72 & 6.0 carats
1998 - The Dickinson/Stevens, 7.28 carats
An end to a mystery
Since 1906, when John Huddleston discovered the first diamonds in this field, people have wondered what was beneath this green soil surface. Just how deep and how large is this diamond deposit?
From 1990 to 1997, Arkansas State Parks and several international commercial mining companies conducted tests to determine the underground size and shape of the diamond deposit, and to evaluate the grade of diamonds from the mine.
Phase I testing determined that this is the world’s 8th largest diamond deposit, in size and shape. It extends over 660 feet into the earth. In phase II, 210 diamonds totaling 45.7 carats were recovered. This verified that a significant number of diamonds occur within this large deposit. However, the grade was less than commercially profitable at the time and interest in further testing diminished.
--------------------------------------------
Geologic provenance: Prairie Creek Lamproite (Prairie Creek Pipe), mid-Albian Stage, late Early Cretaceous, ~106 Ma
Locality: Crater of Diamonds State Park, just south of the town of Murfreesboro, Pike County, southwestern Arkansas, USA (34° 01’ 55” to 34° 02’ 05” North latitude, 93° 40’ 18” to 93° 40’ 25” West longitude)
Diamond (Prairie Creek Lamproite, Early Cretaceous, 106 Ma; Crater of Diamonds, Pike County, Arkansas, USA) 2
Diamond from Arkansas, USA. (longest dimension of crystal = ~1.65 centimeters; USNM R2-00, National Museum of Natural History, Smithsonian Institution, Washington D.C., USA; public domain photo provided by the USNM)
A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 6100 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.
Elements are fundamental substances of matter - matter that is composed of the same types of atoms. At present, 118 elements are known (four of them are still unnamed). Of these, 98 occur naturally on Earth (hydrogen to californium). Most of these occur in rocks & minerals, although some occur in very small, trace amounts. Only some elements occur in their native elemental state as minerals.
To find a native element in nature, it must be relatively non-reactive and there must be some concentration process. Metallic, semimetallic (metalloid), and nonmetallic elements are known in their native state.
The element carbon occurs principally in its native state as graphite (C) and diamond (C). Graphite is the common & far less valuable polymorph of carbon. A scarce polymorph of carbon is diamond. The physical properties of diamond and graphite couldn’t be more different, considering they have the same chemistry. Diamond has a nonmetallic, adamantine luster, typically occurs in cubic or octahedral (double-pyramid) crystals, or subspherical to irregularly-shaped masses, and is extremely hard (H≡10). Diamonds can be almost any color, but are typically clearish, grayish, or yellowish. Many diamonds are noticeably fluorescent under black light (ultraviolet light), but the color and intensity of fluorescence varies. Some diamonds are phosphorescent - under certain conditions, they glow for a short interval on their own.
Very rarely, diamond is a rock-forming mineral (see diamondite - www.flickr.com/photos/jsjgeology/14618393527).
The rough diamond seen here is from a Cretaceous-aged lamproite body at Crater of Diamonds State Park in Arkansas. At the site, diamondiferous lamproitic rocks of the Prairie Creek Lamproite are exposed (actually, it’s just a field of rocky dirt). Diamonds are found regularly by tourists. The Prairie Creek Lamproite is an ultramafic pipe that intruded Proterozoic-aged (Grenvillian) basement rocks. Prairie Creek rocks are Cretaceous in age and date to about 106 million years old.
A strange myth has emerged over the years that says Arkansas diamonds are harder than diamonds from elsewhere. This is not true. Diamonds have a hardness of 10 on the Mohs Scale, whether they're from Arkansas or not.
This diamond is somewhat rounded due to magmatic resorption.
--------------------------------------------
From park signage:
THE “CRATER” PIPE
The “Crater” diamond deposits are the result of a series of volcanic eruptions that carried a diamond-bearing magma called peridotite to the surface. This molten rock slowly cooled, forming a solid, vertical shaft or pipe.
Later, secondary eruptions occurred that shattered much of the peridotite. The fragmented mixture left by the eruptions formed a volcanic breccia that comprises the majority of the searching area. The intrusive peridotite, and a tuff and fine-grained breccia, also formed by the eruptions, cover the remainder of the mine surface.
STRATIGRAPHIC PROFILE
The volcanic eruptions forming the “Crater’s” pipe had to force their way upward through the layers of rock and sediment formed during Paleozoic and Mesozoic times.
Diamonds are located throughout the pipe - from the surface to an undetermined depth (possibly several thousand meters). The mine is simply the exposed surface area of the pipe.
TOPOGRAPHY
Surface features at the “Crater” today reflect the depositional, erosional, and weathering processes which have occurred since the pipe’s formation. Erosional forces, such as wind and water, have erased the original surface features of the pipe and surrounding rock formations.
Most of the exposed volcanic rocks tend to weather rapidly forming soils of various colors. It is in these soils that we find diamonds today.
In 1948, Glen L. Martin leased property to build a diamond processing plant. Martin’s plant operation lasted only eight months after recovering less than 250 carats of diamonds. The original location of the Martin Plant has been preserved. Its location lies just south of the Visitor Center.
DAZZLING DIAMONDS FROM THE CRATER
Over 85,000 Crater diamonds have been found. Each year an average of 600 are found.
1924 - The Uncle Sam, 40.23 carats
1956 - The Star of Arkansas, 15.33 carats
1956 - The Eisenhower, 6.11 carats
1960 - The Gary Moore, 6.43 carats
Over 24,000 diamonds have been found since Crater of Diamonds State Park was established in 1972.
1975 - The Amarillo Starlight, 163.7 carats
1978 - The Lamle Diamond, 8.61 carats
1981 - The Star of Shreveport, 8.82 carats
1986 - The Conell Diamond, 7.95 carats
1990 - The Strawn-Wagner, 3.03 carats
1997 - The Cooper Diamonds, 6.72 & 6.0 carats
1998 - The Dickinson/Stevens, 7.28 carats
An end to a mystery
Since 1906, when John Huddleston discovered the first diamonds in this field, people have wondered what was beneath this green soil surface. Just how deep and how large is this diamond deposit?
From 1990 to 1997, Arkansas State Parks and several international commercial mining companies conducted tests to determine the underground size and shape of the diamond deposit, and to evaluate the grade of diamonds from the mine.
Phase I testing determined that this is the world’s 8th largest diamond deposit, in size and shape. It extends over 660 feet into the earth. In phase II, 210 diamonds totaling 45.7 carats were recovered. This verified that a significant number of diamonds occur within this large deposit. However, the grade was less than commercially profitable at the time and interest in further testing diminished.
--------------------------------------------
Geologic provenance: Prairie Creek Lamproite (Prairie Creek Pipe), mid-Albian Stage, late Early Cretaceous, ~106 Ma
Locality: Crater of Diamonds State Park, just south of the town of Murfreesboro, Pike County, southwestern Arkansas, USA (34° 01’ 55” to 34° 02’ 05” North latitude, 93° 40’ 18” to 93° 40’ 25” West longitude)