|Rare Caledonite Crystals. From: Victoria, Pozo Almonte, Iquique Province, Tarapacá Region, Chile|
Credit: Christian Rewitzer
Caledonite is an uncommon mineral found in the oxidized zones of copper-lead deposits. Caledonite, whose name derives from Caledonia, the historical name of its place of discovery (Scotland), is a richly colored blue-green sulfate-carbonate mineral of lead and copper with an orthorhombic crystal structure. Given that caledonite is found in oxidized copper/lead deposits, it is frequently found in association with other copper and lead minerals.
Caledonite is seemingly a difficult mineral to classify in that it has both carbonate anions and sulfate anions. There are however more sulfates than carbonates in its formula and it would therefore seem to make the most sense to place it in the Sulfate Class as opposed to the Carbonate Class. The sulfate ion is also more complex and a stronger electronegative anion than the carbonate anion and mineralogists sometimes classify minerals in terms of their highest complexity and/or electronegativity.
It is the same reasoning for not placing this mineral in the Oxide and Hydroxide Class in deference to its hydroxides. Other sulfates that contain carbonate anions in their formula include rapidcreekite, burkeite, hauckite, hanksite, nakauriite, tatarskite, mountkeithite, jouravskite and thaumasite (which has a silicon in its formula as well). There are several minerals, namely macphersonite, tychite, susannite, schrockingerite, leadhillite, nasledovite, motukoreaite, mineevite and brianyoungite; that contain more carbonates than sulfates and these are generally classified as carbonates; just to confuse the issue.
Caledonite forms small, but well formed and intricate crystals. It has a nice high luster, due to its lead content and a beautiful blue to green color due to its copper content. Caledonite is closely related to and often in association with linarite; PbCu(SO4)(OH)2. Linarite is normally a deeper blue and tabular or prismatic, but with a slanted, non-symmetrical termination. Caledonite and linarite are found in the oxidation zone of copper and lead ore deposits. Both minerals are beautiful and make for outstanding micromountable specimens.
|Caledonite Crystals, Red Gill Mine, Roughton Gill, Caldbeck, Allerdale, Cumbria, England, UK. |
Photo Copyright: Stephan Wolfsried
Properties of Caledonite:
Chemical Formula: Pb5Cu2(SO4)3(CO3)(OH)6
Color is green, light blue, blue to blue-green.
Luster is vitreous to resinous or greasy.
Transparency: Specimens are transparent to translucent.
Crystal System is orthorhombic; 2/m 2/m 2/m.
Crystal Habits include small micromountable prismatic and less often, tabular crystals with a truncation formed by multiple parallel domal and pyramidal faces. Acicular interlocking and radial aggregates are typical. Individual crystals can display a wide range of crystal faces.
Cleavage is perfect in one direction (basal) and poor in two (prismatic).
Fracture is uneven.
Hardness is 2.5 - 3.
Specific Gravity is approximately 5.6 - 5.8 (very heavy for translucent minerals).
Streak is greenish white to bluish-green.
Other Characteristics: Crystals usually striated.
Associated Minerals include leadhillite, cerussite, malachite, linarite, brochantite and anglesite.
Notable Occurrences include the type locality of Leadhills, Lanarkshire, Scotland as well as the fine specimens from the Mammoth-St Anthony Mine, Tiger, Arizona. Other locations include the Defense and Modoc mines and Cerro Gordo Mine, Inyo County and the Blue Bell Mine, San Bernardino, California; Beaver Creek, Utah and Dona Ana County, New Mexico, USA; Cornwall, England; Sardinia, Italy and Chile.
Best Field Indicators are crystal habit, density, color, streak, luster and cleavage.