Sunday, March 23, 2008

A picture of a Borealis in a mineral: Labradorite

Picture: Dual view of a cleavage fragment of labradorite showing the multi-colored labradorescence (schiller effect) and the same area viewed without the color. The color is due to optical interference on the 010 polysynthetic twinning planes (Albite Law).

Next in my blog is tectosilicate "Labradorite" variety: Spectrolite ((Ca,Na)(Si,Al)4O8) in all probabilities named like that in honor of its color spectrum reflected in the tabular crystals as a gift for the eyes. Now, labradorite was named like that thanks to its original locality the Labrador Peninsula. Its beauty combined with its hardness have made the Spectrolite a beloved stone for kitchen tops, and floors.

I remember being in the Telephone company of Saudi Arabia to pay my bill while I lived there back in the 80's, and I was in owe watching such luxury, the whole building was labradorite variety spectrolite, I just couldn't believe it, and at least in those years, it was forbidden to take pictures.

Chemical Formula: (Ca,Na)(Si,Al)4O8
Molecular Weight = 271.81 gm
Empirical Formula: Na0.4Ca0.6Al1.6Si2.4O8
Environment: Magmatic and metamorphic rocks.
Locality: Labrador peninsula, Canada.
Name Origin: Named after its locality.
Synonym: Spectrolite
Crystal system: Triclinic - Pinacoidal
Cleavage: [001] Perfect, [010] Good, [110] Distinct
Color: Colorless, Gray, Gray white, White, Light green.
Density: 2.68 - 2.71, Average = 2.69
Diaphaniety: Translucent to transparent
Fracture: Uneven - Flat surfaces (not cleavage) fractured in an uneven pattern.
Habit: Crystalline - Coarse - Occurs as well-formed coarse sized crystals.
Habit: Granular - Generally occurs as anhedral to subhedral crystals in matrix.
Habit: Striated - Parallel lines on crystal surface or cleavage face.
Hardness: 7 - Quartz
Luminescence: Non-fluorescent.
Luster: Vitreous (Glassy)
Streak: White
Radioactivity: GRapi = 0 (Gamma Ray American Petroleum Institute Units)
Labradorite is Not Radioactive

Have a great weekend, once again in Anaco Venezuela, but ready to back to Houston tomorrow morning from Caracas. In the meantime, feel free to visit my website:

Oscar el Mexicano

A picture of a Borealis in a mineral?: Labradorite var. Espectrolite

Saturday, March 22, 2008

Pinkier than the Pink Panther - Rhodochrosite

I am sure that the pink panther's creator was inspired in rhodochrosite (MnCO3) to get that nice color of the funny French cartoon, but, being French, I may be wrong.

Furthermore, the beauty, the habit, and the variety of crystallization make rhodochrosite one of the prettiest, but is one of the softest existing gems in the world, as its hardness is 3 (in the Moss scale of 1 to 10). It means that it can be scratched with a coin, so be real careful (use paper bills instead)!

The fist place (locality) that comes to mind is the one in the Kalahari desert in South Africa, in which the color is beyond a deep pink to almost light cherry red, simply amazing.

South of the Kalahari Desert on the seemingly endless, hot and dust plains of the Northern Cape in South Africa is the Kalahari Manganese Field. The fact that it is estimated to contain 14 billion tons of manganese, not only represents a great economic wealth to the area, but also a paradise to the mineral collector. One hundred and thirty-five minerals have been identified there, since the commencement of mining in the 1940's. Some of the most spectacular Rhodochrosite specimens come from the N'Chwaning II mine, near Kuruman.

Ok, here is the technical data:

Chemical Formula: MnCO3
Molecular Weight = 114.95 gm
Environment: Commonly forms during oxidation of Mn-bearing ores in a carbonate environment.
Locality: Cavnic (Kapnik), Maramures, Romania
Name Origin: From the Greek rhodon -"rose" and chroma - "color."
Synonym: Dialogite
Crystal System: Trigonal - Hexagonal Scalenohedral

Cleavage: [1011] Perfect, [1011] Perfect, [1011] Perfect
Color: Pinkish red, Red, Rose red, Yellowish gray, Brown.
Density: 3.69
Diaphaniety: Translucent to subtranslucent
Fracture: Brittle - Conchoidal - Very brittle fracture producing small, conchoidal fragments.
Habit: Botryoidal - "Grape-like" rounded forms (e.g.. malachite).
Habit: Columnar - Forms columns
Habit: Massive - Granular - Common texture observed in granite and other igneous rock.
Hardness: 3 - Calcite
Luminescence: None.
Luster: Vitreous (Glassy)
Streak: White
Fluorescence: No

That's it for now, have a Good Friday, and a better Saturday.

Oscar Garcia Shelly.

Saturday, March 15, 2008

Is it a candy? no way Jose, it's a Wulfenite

Yeah, in fact it really looks like a candy, doesn't it? But not so fast my friends, Wulfenite (PbMoO4) may be subject to penalty due to the stiff Clean Air Act mandates, and regulations one of these fine days, as it contains lead and Molybdenum, both chemical elements hazardous to human beings. If you don't believe me, just check on the number of recalls to Chinese-made toys brought back to China, because of their lead content. God help us.

At any rate, secondary mineral Wulfenite is another interesting specimen, found in oxidation and enrichment zones with high saturation of lixiviated chemical elements from their original ore deposit, just like adamite, legrandite, hemimorphite, and goethite.

Wulfenite is abundant in Mexico (Chihuahua and Sonora), and in other areas of the world, such as the state of Arizona, in the USA, and the land of the dioptase: Tsumeb, in Namibia, Africa. Most beautiful crystals come from all these places, just like a contest, you never know which is producing a better looking one for the delight of the most avid mineralogist including yours truly.

Ok, how about if you take a look at the various features of Wulfenite including its name origin as follows:

Chemical Formula: PbMoO4
Molecular Weight = 367.14 gm
Environment: Secondary lead mineral. Forms a series with stolzite.
Locality: Bleiberg, Villach, Kärnten (Carinthia), Austria; Los Lamentos Mine in Municipio of Ahumada, Chihuahua Mexico; also in the San Francisco Mine, near Cucurpe in Sonora, Mexico, and so on.
Name Origin: Named after the Austrian mineralogist, Frantz Xaver von Wulfen (1728-1805).
Crystal System: Tetragonal - Dipyramidal
Cleavage: [101] Imperfect
Color: Orange yellow, Waxy yellow, Yellowish gray, Olive green, Brown.
Density: 6.5 - 7, Average = 6.75
Diaphaniety: Subtransparent to subtranslucent
Fracture: Brittle - Conchoidal - Very brittle fracture producing small, conchoidal fragments.
Habit: Massive - Granular - Common texture observed in granite and other igneous rock.
Habit: Tabular - Form dimensions are thin in one direction.
Hardness: 3 - Calcite
Luminescence: None.
Luster: Resinous - Greasy
Streak: Yellowish white

It's weekend time, I am in Anaco, and it's time to go to have dinner in Venezuela, and I think that maybe a couple or arepas, or cachapas with parchita juice will make it. Bon apetite amigos, have a terrific weekend.

Oscar el Mexicano

Wednesday, March 12, 2008

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

Monday, March 10, 2008

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!!!

Saturday, March 1, 2008

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