Category Archives: The Conglomerate

Hematite

[Guest Author Matt Doyle]

Hematite, famous modernly for the steel-grey jewelry often made from it, is more commonly a rust-red ore when found in mining iron[1].  A iron oxide (Fe2O3), hematite’s name comes from haema, the Greek word for blood[2], and most of its direct historical impact comes from that red form.  While hematite is incredibly common – the most common form of iron ore[3] – even in ancient times it was appreciated for itself, and not only for the metal it could produce.  In modern times, magnets are used to harvest hematite from mine tailings.

hematite1     Red hematite most commonly possesses an earthy luster, appearing anywhere from rust-colored to Powdered red hematite is also known as rouge.  Perhaps most famous as a cosmetic used for centuries to redden the skin, it is the same substance as jeweler’s rouge, used to polish metal and gemstones, and also frequently used to help strop a barber’s straight razor.  Red ochre and yellow ochre painting pigments also owe their color to a mixture of red hematite and clay – unhydrated in red ochre, and hydrated in yellow[4].  Maybe most strikingly, hematite is the basis of red chalk, and red chalk drawings have many prominent places in human history and the history of art, including the sketches of Leonardo DaVinci, the body painting of corpses in paleolithic cultures one hundred and sixty to eighty thousand years ago[5], and numerous cave paintings dating back as much as forty thousand years[6].  Red chalk mines dates back as far as 5000 BCE.

Grey hematite, unlike the “bloodstone” variety that gave it its name, has a metallic luster, and can appear almost like a dark mirror when sufficiently polished.  Faceted, it appears nearly black, and smooth, it has a gray, lustrous tone similar to a black pearl.  Used as a gemstone in jewelry, for gilding, or for carved intaglios, it was especially popular in Victorian England, and is still used today, in part because it is common enough to be relatively affordable. In its more jewel-like form, it has been sought after for over two millennia, since the Etruscans found deposits of it on the island of Elba.

hematite2     Outside of iron mines, hematite is commonly found in banded iron formations, hot springs, clay banks, and other places where iron interacts with water[7] (or more rarely, without water, as a result of volcanic activity).  Whether grey or red, it always leaves a red streak[8] (and a grey stone leaving a red streak is often striking and startling to students in the lab seeing it for the first time). Hematite often contains enough inclusions of magnetite to appear attracted to magnets, however, hematite itself is only weakly ferromagnetic when encountered at room temperature.  Its specific magnetic properties are variable in peculiar ways depending on the scale of the hematite crystal, and its small magnetic moment, as well as the temperatures at which it transitions from antiferromagnetic to paramagnetic, have been the subject of much discussion since the 1950s (and as such, could make up an essay – or many scholarly papers – of their own).

[1] https://en.wikipedia.org/wiki/Hematite

[2] http://www.mindat.org/min-1856.html

[3] http://geology.com/minerals/hematite.shtml

[4] http://www.mineralszone.com/minerals/ochre.html

[5] https://en.wikipedia.org/wiki/Pinnacle_Point

[6] http://www.eurekalert.org/pub_releases/2007-10/asu-rfe101207.php

[7] http://mars.nasa.gov/mer/spotlight/hematite01.html

[8] http://www.minerals.net/mineral/hematite.aspx

Gypsum

The mild-mannered gypsum is not only a pretty sulfate to look at, but is harvested for numerous functions. The crystals are tabular, and often twinned. It can also form massive, granular, and fibrous habits. Radiating forms are called “daisy gypsum,”, and rose-shaped forms “desert rose.” Gypsum tends to be fairly drab with color, varying from near colorless, white, and gray, to a more green, yellow, or reddish hue. Its name comes from the Greek word “gypsos,”, meaning chalk or plaster.

It is mined for use as a fertilizer, plaster, chalk, and sheetrock / gypsum board. The granular form called alabaster is used in carving and sculpture. “Plaster of Paris” is dehydrated gypsum – by adding water back into the powder, the mixture creates an exothermic reaction (gives off heat), and “sets” into a hardened form. This is useful for making casts of objects. The fibrous crystal form is called “satin spar” and “senelite.”

gypsum 3

Selenite – fibrous form

gypsum 4

Alabaster – granular gypsum

gypsum 2

Desert Rose – rosette gypsum

Gypsum has a white streak, but ranges from transparent to opaque. It is a very common mineral found in many locations. It can be deposited from lakes and seawater, hot springs, and other evaporative environments.

**Becky Trivia** Two fossil sites across North Dakota, on opposite ends of the state, hold gypsum. To the east, the Pembina Gorge locality once held a vast inland sea. Gypsum is so plentiful there it is the main mineral replacing the mosasaur and fish fossils, giving them a very soft, fragile form. Just off site, people can wander and pick up satin spar spears ranging from clear to black. To the west, the Whiskey Creek locality was once a swampy environment similar to the everglades. Sheets of gypsum can be found in and around those crocodile fossils as well.

The chemical formula is CaSO4·2H2O, and a hardness of 2 on the Mohs scale.

 

Pellant, Chris. Rocks and Minerals. New York: Dorling Kindersley, 1992. Print. Pg. 110.

http://www.minerals.net/mineral/gypsum.aspx

http://www.mindat.org/min-1784.html

Malachite

malachite3

Fibrous malachite

Malachite is an intense green colored, copper carbonate mineral. Instead of being a solid color, it is often banded in shades of green. While it can form tabular and twinned crystals, it is more often seen as botryoidal masses, or stalactitic, with a fibrous banded structure or crusts. Malachite can commonly be found with azurite. It has a hardness of 3.5-4 – soft enough to carve readily, yet still taking a polish. The stone is used for decoration, ornamentation, and jewelry. It can also be crushed and made into a green pigment. It was originally worn to ward off evil spirits.

Malachite comes from many locations, including Russia, Africa, Australia,

malachite2

Malachite with azurite

Brazil, and Arizona. It was named after the Greek word “mallows”, alluding to its leafy green color. It has also been called Atlas ore and Green Copper.

The chemical formula is: Cu2CO3(OH)2

 

http://www.minerals.net/mineral/malachite.aspx

http://www.mindat.org/min-2550.html

Pellant, Chris. Rocks and Minerals. New York: Dorling Kindersley, 1992. Print. Pg. 105.

Busbey, Arthur Bresnahan. Rocks & Fossils. Alexandria, VA: Time Life, 1996. Print. Pg. 174

Fulgurite

fulguriteNot truly a mineral or a fossil, fulgurite is sometimes referred to as petrified lightning. The word comes from Latin (fulgur = lightning), and is amorphous “lechatelierite” silica glass. When lightning or electricity strikes a conductive surface, the temperature can skyrocket to at least 2,950F and melt surrounding silica or quartz leaving behind a glass tube. Lechatelierite can also be formed when a meteor impacts the ground, or during volcanic explosions.

They vary in shape, size, and color, and can be very fragile. Larger pieces have been recorded between 13-30 feet in length. Their shape mimics the path the electrical charge took.

https://en.wikipedia.org/wiki/Fulgurite

http://geology.utah.gov/map-pub/survey-notes/glad-you-asked/what-are-fulgurites-and-where-can-they-be-found/

Campeloma

campeloma 1Campeloma are common freshwater snails, with an ancestry dating back to the Cretaceous of North Dakota. These gastropods (“stomach foot”) can be found at times with other snail genus, mainly Viviparus or Lioplacodes. Campeloma shells tend to be bulbous, with highly convex body whorls – the spirals that make up the shell, and about an inch long. Viviparus are about the same size as Campeloma, but the whorls are nearly flat on the outside, giving the shell a smooth cone appearance. Lioplacodes has bulbous whorls like Campeloma, but the overall cone-shape of the shell is much more pointy (like a stubby unicorn horn). Like their modern counterparts, these snails most likely lived between 3-11 years. They had a varied diet, including carrion and vegetation.campeloma 2

Septarian nodule (concretion)

External, weathered surface of a septarian nodule.

External, weathered surface of a septarian nodule.

A “concretion” refers to a type of mineral deposit, where layers of precipitated minerals are attracted to a starting nucleus (such as shell or bone) – much like how a pearl forms. Generally this occurs in water-rich environment, where there are minimal restrictions or obstructions to the concretion growth, resulting in a round shape. Numerous concretions may start growing near each other, then merge to form larger masses.

These particular concretions contain angular cracks and cavities. The word “septarian” comes from the Latin word septum, meaning a wall or partition, referring to the cracks riddling through these concretions. It is believed the cracks are formed from shrinkage of material – where the outside deposited material is denser or hard, while the innermost material is softer, which shrinks and cracks over time. These cracks are then filled with precipitate minerals such as calcite or siderite.

Internal polished surface of septarian nodule, showing calcite and iron-rich deposits.

Internal polished surface of septarian nodule, showing calcite and iron-rich deposits.

Septarian nodules are common in the northeastern part of North Dakota, near the Pembina Gorge. As they weather out of the ground, the softer, original concretionary material erodes faster than the cracks, giving the nodule a turtle-shell like appearance. These are often mistaken for fossils.

 

http://en.wikipedia.org/wiki/Concretion

Buchanan, Rex C., Tolsted, Laura L., and Swineford, Ada, 1986, Kansas Rocks and Minerals: Kansas Geological Survey, Educational Series 2, 60 p.

Quartz

quartz 1Quartz is a very common mineral, with numerous shapes and colors. For as long as people have enjoyed shiny things, quartz has been used in jewelry and carvings. Different types of quartz include: chalcedony (white, lightly colored), agate (multi-colored, banded), onyx (agate with straight, consistent bands), jasper (red-brown), aventurine (chalcedony with shimmering inclusions), tiger’s eye (gold to red-brown), amethyst (purple), citrine (yellow to orange), prasiolite (light green), rose (pink), smoky (brown to gray), carnelian (red-orange), and others. The biggest difference in forms is whether the mineral is macrocrystalline (showing individual crystals), or microcrystalline (tiny crystals, visible under magnification). Transparent varieties showing good crystal forms such as amethyst or citrine would be macrocrystalline.

Quartz can be found as a component of many other rocks and minerals – granites, sandstone, and schist, for example. Crystals are generally six-sided, but commonly twin. It is a 7 on the Mohs scale, with a white streak. The chemical formula for quartz is SiO2 . As the second most common mineral found on earth (the most common being feldspar), quartz can be found world-wide.

It is piezoelectric – meaning it generates an electrical charge if put under stress. It is used as an oscillator in radios, watches, gauges, etc. Quartz sand is used to make glass, as well as a sandblasting abrasive.

http://www.mindat.org/min-3337.html

http://www.minerals.net/mineral/quartz.aspx

http://en.wikipedia.org/wiki/Quartz

Orthoceras

External shell

External shell

Orthoceras (meaning “straight horn”) is an extinct animal related to squid and octopus. Unlike tasty calamari, their shells (being in the phylum mollusca) are on the outside of their bodies, instead of the pen-bone or gladius seen today. They are often mistaken for another straight-shelled cephalopod, Baculites, however lived much earlier during the Ordovician through Triassic (488-190 mya), rather than the Cretaceous (145-65 mya). Limestone deposits containing mass die-offs of Orthoceras are common in Morocco. The ones from this location tend to be white in color, with a black background – often used in decorative carvings, bowls, and even countertops.

Their straight shells are divided horizontally by “septa”, which separate living chambers of the

animal. When it grew too large for one chamber, it would add on another. A tube running through the length of the shell, called a “siphuncle”, helped regulate water and air in the shell, allowing for both movement and buoyancy. They vary in size, from tiny centimeter long shells, to more than 6 feet long (more, if you add the tentacles!).

Polished Orthoceras, showing siphuncle and septae.

Polished Orthoceras, showing siphuncle and septae.

http://en.wikipedia.org/wiki/Orthoceras

http://www.fossils-facts-and-finds.com/orthoceras.html

Galena

galena2Galena, a lead sulfide, is not only a source of lead, but of silver as well. Metal extraction is simplified by its low melting point. It is dark grey-silver in color, with an octahedral crystal structure. The metal surface can tarnish when exposed to air, darkening the color. It was named in 77AD by Pliny the Elder, from Greek “galene”, literally meaning lead ore.

Deposits can be found in England, Bulgaria, Australia, the USA, and many North African countries, among others. It is the State Mineral of Missouri and Wisconsin.

This mineral is used, and has been used, for a variety of applications. From kohl in ancient Egypt, lead shot, green glazes for pottery, and crystal radio sets. It has a variety of common names, such as “blue lead ore”, and “potter’s ore”.galena1

The chemical formula for Galena is PbS, and it has a hardness of 2.5-3 on the Mohs scale. Being mainly lead, it feels very heavy when held. Its specific gravity is 7.4-7.6, whereas pure lead is 11.3. Pyrite, an iron sulfide, has a specific gravity of 4.9-5.2.

http://en.wikipedia.org/wiki/Galena

http://www.mindat.org/min-1641.html

http://www.minerals.net/mineral/galena.aspx

Palache, Charles, Harry Berman & Clifford Frondel (1944), The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana Yale University 1837-1892, Volume I: Elements, Sulfides, Sulfosalts, Oxides. John Wiley and Sons, Inc., New York. 7th edition, revised and enlarged, 834pp.: 200-204.

Amethyst

amethyst2Amethyst is a purple variant of quartz (yellow = citrine, green = prasiolite), with its color coming from iron impurities. Its name comes from the Greek “a-“ (not) and “methystos” (intoxicated), stemming from the belief that the stone protected against intoxication. It was also thought that the stone would make a person more clear-headed. Amethyst can be found world-wide, with large deposits found in Brazil, South Korea, Austria, Russia, and numerous other countries. Before large deposits were found, this stone used to be lumped with diamonds, rubies, and sapphires for its rarity and preciousness. With a hardness of 7 on the Mohs scale, it makes a good gemstone – fortunate, as it is the birthstone for February. Cutting the stone properly can be challenging. The crystals do not often have a uniform concentration of color, but vary in shades of purple and mauve. These crystals are often found in geodes – some of which are massive in size.amethyst1

Chemical Formula: SiO2

Kunz, George F. The Curious Lore of Precious Stones. New York City: Halcyon House, Blue Ribbon, 1930. Print.

http://www.mindat.org/min-198.html

http://www.gemdat.org/gem-198.html

http://www.gemstone.org/index.php?option=com_content&view=article&id=98:sapphire&catid=1:gem-by-gem&Itemid=14