Eglestonite

Where it forms, where it's found

Type locality: Terlingua Mining District
Texas
USA

30recorded occurrences

Source · OpenStreetMap

Safety & handling

Physical

Hardness: 1Talc
2Gypsum
3Calcite
4Fluorite
5Apatite
6Orthoclase
7Quartz
8Topaz
9Corundum
10Diamond
Lustre: Adamatine · resinous
Transparency: Translucent
Colour: Yellow · orange-yellow · brown · turns dark brownish · then black · on exposure to light · yellow-brown to brown (transmitted light)
Streak: Yellow to greenish-yellow
Tenacity: brittle
Fracture: Irregular/Uneven · Conchoidal
Density: 8.33 g/cm³

Optical

Optical type: Isotropic
Notes: Exhibits weak anomalous birefringence at times. Blackened immediately by hydrogen sulfide or ammonia. Decomposed by acids with separation of Calomel.

Isotropy testPPL ↔ XPL diagnostic

PPL intrinsic colour; no change on stage rotation

XPL extinct at every orientation

Crystallography

Crystal system: Isometric
Space group: #225
Cell parameters: a = 16.03 Å
Morphology: Dodecahedral, with striations parallel to the edges, and elongated on [001] at times, into almost hair-like forms. Octahedral and cubic forms less common. Massive; as crusts.
Type-locality form: Dodecahedral crystals, < 1 mm, isolated or in a loose crust.

Crystal structure

Chemical composition

Constituent elements: Mass composition breakdown
Element Atoms At. mass g/mol Mass g/mol Mass share
80HgMercury Mercury 6 200.592 1203.552
89.62%
17ClChlorine Chlorine 3 35.450 106.350
7.92%
8OOxygen Oxygen 2 15.999 31.998
2.38%
1HHydrogen Hydrogen 1 1.008 1.008
0.08%
Total 1342.908 100.00%
Mass share = atoms × atomic mass ÷ molar mass × 100
From IMA formula

Mass composition breakdown
Element	Atoms	At. mass g/mol	Mass g/mol	Mass share
80HgMercury	Mercury	6	200.592	1203.552	89.62%
17ClChlorine	Chlorine	3	35.450	106.350	7.92%
8OOxygen	Oxygen	2	15.999	31.998	2.38%
1HHydrogen	Hydrogen	1	1.008	1.008	0.08%
	Total	1342.908	100.00%

Synonyms

Eglestoniet

In other languages

German: Eglestonit
Spanish: eglestonita
Italian: Eglestonite
Chinese: 褐氯汞矿

Classification

Strunz

10th ed.

3.DD.05

3HalidesClass
3.DOxyhalides, hydroxyhalides and related double halidesDivision
3.DDWith HgGroup
3.DD.05EglestoniteSpecies

Dana

8th ed.

10.05.04.01

10Oxyhalides and HydroxyhalidesClass
10.05A_m(O,OH)_pX_qType
10.05.04— unnamed intermediate level —Group
10.05.04.01EglestoniteSpecies

CIM

—

8.5.6

8Halides - Fluorides, Chlorides, Bromides and Iodides; also Fluoborates and FluosilicatesClass
8.5Halides of Zn and HgGroup
8.5.6EglestoniteSpecies

Group, growth & confusion

5 minerals

Literature, links & citation

Citations

1903Moses, A.J. (1903) Eglestonite, terlinguaite, and montroydite, new mercury minerals from Terlingua, Texas. American Journal of Science: 16: 253-263.
1904Moses, A. J. (1904) Eglestonit, Terlinguaït und Montroydit, neue Quecksilbermineralien von Terlingua in Texas. Zeitschrift für Kristallographie, 39 (1). 3-13 doi:10.1524/zkri.1904.39.1.3DOI: 10.1524/zkri.1904.39.1.3
1909Hillebrand, W.F.; Schaller, W.T. (1909) The mercury minerals from Terlingua, Texas. Bulletin 405. US Geological Survey p.1-174. doi:10.3133/b405 DOI: 10.3133/b405
1911Rogers, A.F. (1911) Eglestonite from San Mateo County, California. American Journal of Science: 32(187): 48-50.
1932Bird, Paul H. (1932) A new occurrence and x-ray study of mosesite. American Mineralogist, 17 (12) 541-550

Cite this entry

@misc{mineral2026,
  author    = {Mineral Index editorial board},
  title     = {Eglestonite — Mineral Index},
  year      = {2026},
  url       = {https://mineralindex.org/minerals/eglestonite-1357},
  note      = {Accessed 2026-05-11}
}