Category Archives: The Conglomerate

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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