A CUP OF CUPRITE

A rockhound getting to know you over a cup of lager: Are you full of  beryllium, gold, and titanium, because you are Be-Au-Ti Full.

Well, one day moves on to another when one is sequestered in your home.  But, as previously stated I am well and safe and often think about those who have lost shelter, food, and a job.  I also thank those medical workers who dedicate their careers, and lives, to protecting others.  For now, I just continue to move from one day to the next with much reading and writing.

Winter arrived again in Colorado Springs with 8 inches of snow and a record low temp of 7 degrees.  Of course, that seems warm to Leadville’s -7 degrees.  Oh well, the sun and warmth will return. 

I have been rummaging around in my minerals and came across some copper oxides and decided to explore their origin in a bit more detail.  So, here goes. 

Minerals, the chocolate chips in the cookies of life.

The oxide minerals are highly desired by many rockhounds as many are colorful with visible crystals.  They usually form in the oxide zone of metallic ore deposits as a result of chemical decomposition of the primary sulfide ore minerals.  This decomposition is the result of groundwater, surface water, oxygen and carbon dioxide causing a chemical change in the unstable sulfides.  Some of the products are the oxide minerals where the oxygen anion with a 2- oxidation state combines with metal cations (positive oxidation state).  One oxide that often has nice colors and beautiful crystals are the copper oxides.


Copper can combine with oxygen in a couple of different ways : copper (I) oxide and copper (II) oxide but also known as cuprous oxide and cupric oxide.  Oxygen, the negative anion, bonds with metal cations by accepting two of their electrons. In cuprous oxide (copper I) two different atoms of copper each donate one electron and the chemical formula is Cu₂O, also known as the mineral cuprite.  In cupric oxide only one atom of copper donates two electrons and the formula is CuO, known as the mineral tenorite. The bonding in copper oxide is ionic--an electrostatic attraction between the positive and negative ions.

Public Domain.  Artist unknown.

Copper is Element 29 on the Periodic Chart of the Elements with copper containing 29 electrons (negative) in outer shells and 29 protons (positive) in the nucleus.  The “normal” electron configuration is 2, 8, 18, 1 as shown above.  Notice that there is a single electron in the outer shell.  When oxygen combines with that single lonely electron the result is Cu₂O or cuprous oxide or cuprite—copper with a 1+ oxidation state combining with one atom of oxygen with a 2- oxidation state so you need two coppers, each with a 1+ oxidation state.  In cupric oxide, tenorite, the oxygen not only takes the lonely outer shell electron to the dance but borrows a second electron from the next orbit and the formula becomes CuO.  Cuprous oxide then has a monovalent cation (the copper) but in cupric oxide where the oxygen borrows 2 electrons the cation copper is a divalent cation.  The cuprous oxide is very stable since there are no open slots on that full penultimate orbit ring (filled and half-filled shells are the most stable). 


Cuprite is one of the best-known copper minerals due to its often-dark red color and octahedral, cubic, or dodecahedral crystals (Isometric Crystal System); however, the red is often so dark that crystals appear black. Cuprite is soft at 3.5-4.0 (Mohs), is very brittle with a conchoidal fracture and a luster that ranges from adamantine to earthy.  It has a brownish-red streak and is transparent to translucent in thin sections. On prolonged exposure cuprite crystals lose their luster and become gray to gray black in color.

Diagram octahedron crystal.

Above three photomicrographs showing dark red octahedrons of cuprite, each crystal less than 1 mm in width.

Extremely small, but very colorful, cuprite crystals on native copper..  Width FOV top: ~8 mm. Width FOV middle: ~4 mm. Width FOV bottom: ~6 mm.

Cuprite is a secondary mineral resulting from the oxidation of primary copper sulfide minerals such as bornite and chalcopyrite.  My specimen is from the Ray Mine northeast of Tucson in Pinal County, Arizona.  Mineralization at the Mine is a porphyry copper deposit (see Posting March 2, 2015).

Needle like crystals of cuprite v. chalcotrichite. Width FOV both ~1.0 cm.  I presume the matrix is goethite/limonite.

I have two other specimens from the Ray Mine are a variety of cuprite called chalcotrichite.  Here the crystals ate not octahedral but greatly elongated capillary or needle like forms.  I really don’t understand about the formation, or the why, of these elongated crystals and had difficulty in locating a good reference.  For those of you interested check out the 1983, vol. 68 of the American Mineralogist, page 790 ff: A TEM study of fibrous cuprite (chalcotrichite); microstructure and growth mechanisms.  Trying to abstract that article for the Post is above my pay grade.  Online at: http://www.minsocam.org/msa/collectors_corner/arc/cuprite.htm.

Photomicrograph of banded chalcedony and tenorite.  Note botryoidal calcite in upper left quadrant.  Width of photo ~1.2 cm.

Blue chalcedony and black tenorite.  Width of photomicrograph ~1.2 cm.

Photomicrograph banded chalcedony left grading into blue chalcedony or silica infused chrysocolla surrounding black tenorite.  Notice green ?chalcedony encased in the blue.  Width of photo ~1.2 cm.

Cupric oxide is recognized in the mineral world as tenorite, a black copper oxide (CuO).  Tenorite would not be an impressive mineral with its earthy to dull to metallic luster and generally massive habit without common accompanying friends—colorful chrysocolla, malachite and azurite.  It is opaque with a black streak and is brittle, commonly with a conchoidal fracture.  Tenorite is soft at ~3.5 (Mohs).  I have only observed massive and botryoidal tenorite; however, some localities produce small crystals (Monoclinic Crystal System).  It appears, from my reading, that visible crystals are only formed when tenorite is the product of volcanic sublimation (crystallized from gasses around volcanic vents).  In fact, the type locality for tenorite is around Mt. Vesuvius in Italy.

This year in Tucson I picked up another rather uncommon oxide containing copper—and iron, the mineral delafossite, CuFeO_2.    It is a combination of the cations cuprous copper (1+)  and ferrous iron (3+) and two atoms of oxygen (4-) or C2¹⁺Fe³⁺O^2-₂.  It is almost equal parts of iron and copper plus two parts oxygen, 1:1:2.  delafossite is black in color, has a hardness ~5.5 (Mohs), a black streak, a metallic luster, and is opaque.  Individual crystals are tabular to equant and at times appear as individuals on a matrix (often goethite); however, in many specimens (such as mine) the individual crystals are massed together in spherulitic masses.  In other instances, the crystals are essentially indistinguishable and massive.  As with cuprite and tenorite, delafossite occurs in the secondary zone of copper deposits (copper porphyry) as an oxidized mineral. It is interesting to note that the specimen of delafossite was once in the collection of Arthur L. Flagg, one of the best-know mineral collectors in Arizona (1883-1961).

Scattered spherical "clumps" of submillimeter delafossite crystals.

A bubble-like surface of delafossite with no visible crystals (at least at the magnification).  Perhaps cuprite is lower left quadrant. Width FOV ~1.0 cm. 

Light gray area is a mass of individual submillimeter delafossite crystals while the dark areas represent the spherical "clumps" of crystals.