Proustite - As red as Hugo Chavez' revolution

Proustite, tricky reading of such a beautiful mineral that consists of sulfur, arsenic and silver (Ag3AsS3). However, its color as red as Hugo Chavez' flag is as magnificent as a rubi, or as a red garnet (pirope), or as a cuprite, a beauty just out of this world. Just see the picture on your right, which by the way, is a specimen from Chañarcillo, in Atacama, Chile. Among other features, the rarity of proustite is that being a sulfide, the crystals are not metallic, but semi-metallic, and glassy and weak as a wulfenite, or a rodochrosite.

Fine proustite crystals, with their transparency, luster and color, are very attractive mineral specimens. However, just as with other silver minerals, it is sensitive to light, and it can form a white coating upon light exposure. This coating can be cleaned off, but fine specimens should be stored in closed containers with limited or no light exposure.

Proustite is usually found in the same ore veins with pyrargyrite (consisting of sulfur, antimony and silver), and other silver sulfides. Proustite and Pyrargyrite are iso-structural, in other words, both minerals have the same structure but with a different chemistry (proustite has antimony, and pyrargyrite has arsenic).

Chemical Formula: Ag3AsS3
Molecular Weight = 494.72 gm
Composition: Silver 65.41%; Arsenic 15.14%; Sulfur 19.44%
Environment: Late forming mineral in hydrothermal deposits, in the oxidized and enriched zone, associated with other silver minerals and sulfides.
Locality: Himmelsfurst mine, Erbisdor, near Freiberg, Germany; Chañarcillo mine in Atacama, Chile; Chihuahua, Mexico.
Name Origin: After the French chemist, J. L. Proust (1755-1826).
Crystal System: Trigonal - Hexagonal Scalenohedral
Cleavage: None
Color: Vermilion, Reddish gray.
Density: 5.5 - 5.6, Average = 5.55
Diaphaniety: Transparent to translucent
Fracture: Brittle - Sectile - Brittle fracture with slightly sectile shavings possible.
Habit: Blocky - Crystal shape tends to be equant (e.g. feldspars).
Habit: Crystalline - Poor - Occurs primarily as crudely formed crystals.
Habit: Massive - Uniformly indistinguishable crystals forming large masses.
Hardness: 2-2.5 - Gypsum-Finger Nail
Luminescence: None.
Luster: Sub Metallic
Magnetism: Nonmagnetic
Streak: Vermilion redHugo Chavez

Greener than an emerald? no, but...prettier? oh yes - Dioptase

(Upper picture: Dioptase on dolomite crystals. Lower picture: Emerald).

It was just a matter of walking through some of the many displays in the 1979 Tucson Gem & Mineral Show, that I had already spent (mentally) around $2 million dollars in specimens, many of which simply took my breath away.

It was the first time I had the opportunity to attend this magnificent world class event, and my first time in the United States, what a great trip. Flew in an American Airlines Boeing 707 Mexico City-Tucson via Dallas, TX, my English was lousy, but I managed to get JIT (just in time) for the connection. Once in Tucson, Arizona, all was easier, nicer, got to the show so excited I wanted them all. But this time, though I dedicate the remembrance to the dioptase, (even its name is lovable). I met in Tucson with my college Mineralogy Professor Dr. Raul Ortiz Asiain and his daughters (Violeta, Magnolia, Dalia and China), all mineral experts, so we had a good time talking about minerals day and night for 3 days in a row eating McDonalds hamburguers on a non-stop basis either.

Didn't bring much money, about $300 dollars, even though it was sufficient to buy some, my budget was limited, because I still had to pay for hotels, meals (McDonalds), and transportation. One of the specimens I bought in the show ($10 US) was a superb nice combination of copper silicate (dioptase) on dolomite, both minerals of the same crystal system (Trigonal - Rhombohedral). The beauty of those cyclosilicate dioptase crystals is not easy to describe, other than its dark blue green, glassy in luster, transparent, well-formed coarse crystals features; I'd say this specimen is prettier than many emeralds I've seen, (which is too adventurous for some), but I admit my weakness for the dioptase, a mineral I thought it was an emerald, but ended up being originally from Tsumeb, Namibia, and not an emerald, but a dioptase. This is the story:

Late in the 18th century, copper miners in Kazakhstan thought they've found an emerald deposit. They found cavities in quartz veins in a limestone, filled with thousands of lustrous emerald-green transparent crystals. The crystals were sent to Moscow, Russia for analysis. However the mineral's inferior hardness of 5 compared with emerald's greater hardness of 8 easily distinguished it. Later, in 1797, René Just Haüy (famous French mineralogist) determined that the "emerald-like" Kazakhstani mineral was in fact not emerald, but a new mineral in science and named it dioptase (Greek, dia, "through" and optima, "vision"), as a consideration to the two cleavage directions of the dioptase, visible inside fine crystals.

Ok, this is the technical data of Dioptase:

Chemical Formula: CuSiO2(OH)2
Molecular Weight = 157.64 gm
Environment: Secondary mineral in oxidized zones of copper deposits.
Locality: Tsumeb and Cochab, Namibia; Altyn Tube, Russia.
Name Origin: From the Greek, dia - "through" and optomai - "vision."
Crystal System: Trigonal - Rhombohedral
Cleavage: [1011] Good
Color: Dark blue green, Emerald green, Turquoise.
Density: 3.28 - 3.35, Average = 3.31
Diaphaniety: Transparent to translucent
Fracture: Conchoidal - Fractures developed in brittle materials characterized by smoothly curving surfaces, (e.g. quartz).
Habit: Cryptocrystalline - Occurs as crystals too small to distinguish with the naked eye.
Habit: Crystalline - Coarse - Occurs as well-formed coarse sized crystals.
Habit: Massive - Uniformly indistinguishable crystals forming large masses.
Hardness: 5 - Apatite
Luminescence: None.
Luster: Vitreous (Glassy)
Magnetism: Nonmagnetic
Streak: Green

I wish you a terrific week, trouble free, and productive. In the mean time, till' my next posting, I hope you learned a bit more about the wonderful mineral world.

Best regards,

Oscar el Mexicano

PD: By the way, my return from that 1979 first visit to the Gem & Mineral Show in Tucson was via Aeromexico that used to stop in Hermosillo, Cd. Obregon, Culiacan, Guadalajara and finally Mexico City, an almost 8 hrs. trip!!!

Violets are blue, and so are you: Amethyst!

Isn't that amazing that some minerals can be found in so many different forms, colors, varieties, locations and ore deposits? such is the case of Tectosilicate Quartz (SiO2), an old German / Saxony's word that meant ice very frozen.

Quartz is the most abundant mineral in the surface of the world, found in almost each and every type of rock, or by itself in geodes, or cavities in acidic volcanic lavas, or magmas. However, one of the prettiest of all quartz varieties is the purple or violet colored gemstone, known as "amethyst" a semi-precious stone beloved by many people, including me.

For a mineralogist, the value of an amethyst comes from its degree of crystallization, color and size. The deeper the color, the larger the size, and the most perfect its crystals are, among other features makes it more valuable, and even appreciated in a Museum of Natural History.

The name "Amethyst" comes from the Greek "amethos" or alcohol. The following story is not well known, but I learned it from Professor Raul Ortiz Asiain (for me, the best Mineralogist in Mexico of all times), that people that eat amethyst dust did not suffer of hang over after drinking heavily the night before. I never tried such a recipe, because first of all I don't drink; I haven't heard this anywhere else in the world, and Professor Ortiz Asiain passed away more than 10 years ago, rest in peace.

Anyhow, let us go technical:

Chemical Formula: SiO2
Molecular Weight = 60.08 gm
Environment: Sedimentary, metamorphic, and igneous rocks.
Locality: Very common mineral found world wide.
Name Origin: From the German "quarz", of uncertain origin.
From the same chemical formula:

Agate - Banded variety of chalcedony
Amethyst - Purple
Avanturine - Feebly translucent chalcedony
Carnelian - Flesh red chalcedony
Cat's Eye - Chatoyant
Chalcedony - Micro crystalline quartz
Chert - Crypto crystalline quartz
Chrysoprase - Apple green chalcedony
Citrine - Yellow
Flint - Micro crystalline quartz
Hornstone - Flint
Jasper - Red or brown chalcedony
Moss Agate - Variety of chalcedony
Plasma - Green chalcedony
Prase - Leek green chalcedony
Rock Crystal - Transparent
Rose Quartz - Rose colored
Sapphire Quartz - Blue colored
Smoky Quartz - Brown to black
Tiger Eye - Pseudomorph of asbestos

Crystal System: Trigonal - Trapezohedral
Cleavage: [0110] Indistinct
Color: Brown, Colorless, Violet, Gray, Yellow.
Density: 2.6 - 2.65, Average = 2.62
Diaphaniety: Transparent
Fracture: Conchoidal - Fractures developed in brittle materials characterized by smoothly curving surfaces, (e.g. quartz).
Habit: Crystalline - Coarse - Occurs as well-formed coarse sized crystals.
Habit: Crystalline - Fine - Occurs as well-formed fine sized crystals.
Habit: Druse - Crystal growth in a cavity which results in numerous crystal tipped surfaces.
Hardness: 7 - Quartz
Luminescence: Triboluminescent.
Luster: Vitreous (Glassy)
Streak: White

Ok, enough I think, by now I am sure you are an expert in Amethyst and its mother quartz. By the way, the only joke I remember about quartz goes like this: Dad (a little quartz talking to his father), is it true that we precipited out of solution? Dad answers: Of Quartz!

Have a terrific weekend, and in the meantime, you may want visit my website where you can find a nice picture of the day taken by me somewhere in the world with my nice Nikon D40X: Oscar el Mexicano