Monday, February 25, 2008

A mineral, or lemon pie filling? - Adamite

Adamite (Zn2(AsO4)(OH) (or hydroxide zinc arseniate) is another beautiful mineral that is found in several locations around the world (Mexico, Chile, Namibia, etc.) in ore deposits rich in zinc, lead, and arsenic that have suffered oxidation, due to weathering and lixiviation, out of the percolating water. All the sulfides decompose, and the resulting rich-in-chemical elements low-temperature solution deposits near and beneath the original ore. One of the formed new minerals is a shiny and crystallized piece of art with a green color that varies considerably. The most beautiful specimens I have had the chance to collect come from the Ojuela Mine in Mapimi, state of Durango. Adamite is found hereby associated with hemimorphite, legrandite, goethite, limonite, cobalt-adamite (purple), calcite, pyrite, cuprite, malachite, hematite, and others I am sure I don't remember. If someone asked me to make does an Adamite crystal looks like to me, I'd say it does look like filling of a lemon pie, the one it used to be cooked from scratch by my late Grandmother in Guadalajara (fattening, but out of this world!).

Chemical Formula: Zn2(AsO4)(OH)

Zinc 45.61 % Zn 56.78 % ZnO
Arsenic 26.13 % As 40.08 % As2O5
Hydrogen 0.35 % H 3.14 % H2O
Oxygen 27.90 % O

Molecular Weight = 286.71 gm
Locality: Ojuela Mine, Mapimi Durango Mexico; Chile, Atacama, Chañarcillo
Name Origin: Named after the French mineralogist Gilbert Joseph Adam (1795-1881).

Crystal System: Orthorhombic - Dipyramidal
Cleavage: [101] Good, [010] Poor
Color: Yellow, Green, Violet, Pink, Yellowish green.
Density: 4.3 - 4.5, Average = 4.4
Diaphaniety: Subtransparent
Fracture: Brittle - Generally displayed by glasses and most non-metallic minerals.
Habit: Druse - Crystal growth in a cavity which results in numerous crystal tipped surfaces.
Habit: Encrustations - Forms crust-like aggregates on matrix.
Habit: Tabular - Form dimensions are thin in one direction.
Hardness: 3.5 - Copper Penny
Luminescence: Fluorescent and phosphorescent.
Luster: Vitreous - Resinous
Streak: white
Fluorescent: Yes, crystals fluoresce green under SW ultra violet light

I hope you have a productive week, visit if you have a chance my updated website: Oscar el Mexicano, meantime, see you in my next posting.

Saturday, February 9, 2008

Fool's gold

I was around 9 years old, when I saw the first pyrite specimen in my life, and I remember I was so greatly impressed, that I wanted to go to the site where the person that showed it to me (a neighbor by the name of Javier Garcia) said he had collected the piece from, the "Cerro del Cuatro" (the hill of the four) in Guadalajara, state of Jalisco in Mexico.

From that day, I fell in love with minerals; and soon enough, my dad discovered I had a new hobby, and bought a Mineralogy booklet for the family, which contained, among others, some pictures of perfect pyrite cubes crystals from Rio Tinto, Spain. I was hooked, and eager to learn more and more about pyrite and other minerals. Pyrite was, at any rate, the mineral that changed my life, with its luster and a perfect crystallization. I would've probably be a doctor, or a pilot, but not a geologist otherwise, thanks to Pyrite.

One of the first things I learned about pyrite was that its origin was related to volcanoes, and that it had a funny nickname: "fool's gold," due to its brassy yellow color, and metallic luster resembling gold to some. How many people bought pyrite in lieu of gold just to find later on they have been fooled? maybe a good bunch, but such is life.

Anyhow, let's see what is this fool's gold or pyrite consist of:


Chemical Formula: FeS2
Composition: Iron (Fe) 46.55%; Sulfur (S) 53.45%
Molecular Weight = 119.98 gm
Empirical Formula: Fe2+S2 (Iron sulfide)
Environment: Sedimentary, magmatic, metamorphic, and hydrothermal deposits.
Locality: I collected beautiful crystals from Naica, Chihuahua, and Fresnillo, Zacatecas, as I used to work for Cia. Fresnillo, SA between 1977 and 1980. But pyrite is common on a world wide basis, related to almost any kind of ore deposit.
Name Origin: From the Greek, pyros - fire and lithos - stone or mineral, "stone which strikes fire," in allusion to the sparking produced when iron is struck by a hammer, or by a lump of pyrite

For more technical details, please refer to:

In the meantime, have a great weekend, see you in my next posting.


Oscar Garcia Shelly

Sunday, February 3, 2008

There are only four precious stones in the world!

There are only four (4) precious stones in the world, contrary to what many people believe. The word precious has evidently two meanings - and both being adjectives - can definitely confuse even to the most knowledgeable person. The funny thing about it, is that some varieties of these precious stones may be as pretty, and even more expensive than a precious stone. The value of these specimens is a function of their purity, quality, color, perfection of its crystals, size, durability (thanks to their hardness), and so forth. So don't be surprised next time you see an opal from Coober Pedy Australia in or around thousands of dollars.

In summary, these are the only four precious stones in the world, everything else even though look precious, are semiprecious stones. Of course, for the mineralogist, and experience rock collector, there are also semi-ugly and very ugly stones (good to practice pitching). What makes them gain such qualification is the opposite of the adjectives of a precious stone. Following are the only four precious stones in the world:

Diamond (multi color, sapphire (blue), emerald (green) and ruby (pink to red). Following is the description of each and every one.


Chemical Formula: C
Composition: Carbon 100.00%
Molecular Weight = 12.01 gm
Environment: Gas rich, ultra-basic diatremes from mantle depths (>30 km), and alluvial placer deposits derived from the Kimberlite (in honor of Kimberley, South Africa) rocks. Kimberlite consists of olivine, garnet, pyroxene, and calcite.
Localities: Kimberly, republic of South Africa. India. Brazil. Ural Mountains, Russia. Murfreesboro, Arkansas, USA.
Name Origin: From the Greek, adamas, meaning "invincible" or "hardest."


Chemical Formula: Al2O3
Composition: Aluminum 52.93% and Oxygen 47.07%
Empirical Formula: (Al2O3)
Environment: Contact and regionally metamorphosed rocks.
Locality: Tchainit and Yakutia, Russia.
Name Origin: Probably derived from the Sanskrit, kuruvinda, meaning "ruby."
Synonym: Leucosapphire - colorless (I do not agree, but that is what you find in the Dana classification). If it does not have the beautiful blue color, it's just plain and simple: is everything, but a sapphire!

Soon Emerald and Rubi.

In the meantime, have a wonderful weekend, the remaining of it, and enjoy the super bowl.

Best regards,

Oscar G. Shelly