Editorial Feature

Calomel – Occurrence, Properties, and Distribution

Jun 11 2014

Calomel was named after the Greek words for 'beautiful' and 'black'. Although the mineral is white, the black refers to its reaction with ammonia, which gives a beautiful black coloration.

Properties of Calomel

The following are the key properties of calomel:

Cell Data
- Space group: I4/mmm
- a = 4.4795(5)
- c = 10.9054(9)
- Z = 4

Crystal Data
- Tetragonal
- Point group: 4/m2/m2/m
- Variable and complex habit
- Crystals: commonly tabular on {001}, prismatic || [001], equant pyramidal, to 1cm
- In drusy crusts, earthy, massive
- Twinning: On {112}, contact and penetration twins, may be repeated
- X-ray powder pattern: 3.170 (100), 4.15 (75), 2.067 (40), 2.727 (30), 1.962 (30), 1.970 (16), 2.824 (12)

Optical Properties
- Optical class: Uniaxial (+)
- Pleochroism: Weak
- Absorption: E > O
- ω = 1.973
- ε = 2.656

Estimated Properties

Electron density	Bulk density (electron density) = 5.98 g/cm³ note: Specific gravity of calomel = 7.27 g/cm³
Photoelectric	PE_Calomel = 1,464.64 barns/electron U = PE_Calomel x ρ electron density = 8,751.40 barns/cm³
Fermion index	Fermion index = 0.25 Boson index = 0.75
Radioactivity	Calomel is not radioactive

How to Identify Calomel

Calomel can be identified in the field by its color variations, such as white, yellowish-gray, gray, yellowish-white, and brown. Its translucent to sub-translucent form has {100} poor and {011} poor cleavage. This mineral has an adamantine-resinous luster, with pale yellowish-white streak. The fracture on this mineral is sectile, meaning curved shavings or scrapings produced by a knife blade.

The density of calomel is 6.4 to 6.5 g/cm³, with a hardness of 1.5 to 2 - between talc and gypsum. It is fluorescent.

Global Distribution

Calomel is distributed in the following places:

Germany - From Moschellandsberg, Rhineland-Palatinate
Spain - At Alva, Serbia Large crystals from Almaden, Ciudad Real Province
France - Found near Montpellier, Herault
England - At the Chatsworth mine, Grassington Moor, Yorkshire
Russia - From the Kadyrel deposit, Tuva, Siberia
USA - In California, near Palo Alto, San Mateo Co. and the Redington mine, Napa Co. Also from the Sunflower district, Maricopa Co. Arizona; and in the McDermitt mine, Humboldt Co. Nevada; abundant at Terlingua, Brewster Co. Texas; and from near Jackfork, Pike Co. Arkansas
Mexico - Rich specimens from El Doktor, Queretaro; Huahuaxtla, Guerrero; Plateros, Zacatecas

Occurrence of Calomel and Useful Mineral Association

Calomel occurs in an uncommon secondary mineral, formed through the alteration of other mercury minerals.

It is often associated with minerals such as mercury, amalgam, cinnabar, mercurian tetrahedrite, terlinguaite, montroydite, kleinite, eglestonite, moschelite, calcite, kadyrelite, kuzminite, limonite, chursinite, kelyanite, and clay minerals.