Mineral Information and Data:

MgTiO3 [CNMNC approved formula]

Named in honor of Sir Archibald Geikie (1835–1924), Scottish geologist and Director-General of the Geological Survey of Great Britain.

IMA status : Valid species (Pre-IMA 1892) Grandfathered
(8th edition) :  
  (04) Simple Oxides
  (04.03) with a cation charge of 3+ (A+++2 O3)
  (04.03.05) Ilmenite Group
(10th edition) :
  (04) Oxides and Hydroxides
  (04.C) Metal:Oxygen = 2:3, 3:5, and Similar
  (04.CB) With medium-sized cations
Crystal system: Trigonal System
Point group (H-M): 3 — rhombohedral
Space group: R3
Unit cell: a = 5.05478(26) Å, c = 13.8992(7) Å, Z = 6
Crystal Habit: As tabular and prismatic crystals, showing {0001}, {1011}, {2025}, {0221}, to 2.5 cm; more commonly subhedral to anhedral, and as irregular to rounded grains.
Color: Black, less commonly ruby-red; in transmitted light, brownish to purplish red; gray in reflected light, with red internal reflections
Diaphaneity: Opaque to translucent
Luster: Submetallic
Hardness (Mohs): 5 - 6
Measured Density: 3.79 – 4.2 g/cm3
Cleavage: {1011}
Fracture: Conchoidal to subconchoidal
Streak: Purplish brown
Polymorphism and Series: Forms a series with ilmenite.
Geologic Setting: Forms during contact metamorphism of impure magnesian limestones. Also in carbonatites, kimberlites, serpentinized ultramafic rocks, and in gem-bearing placer deposits.
Mineral Association: Rutile, spinel, clinohumite, perovskite, diopside, serpentine, forsterite, brucite, hydrotalcite, chlorite, calcite.


Origin: Monte dei tre Abati, Coli, Piacenza Province, Emilia-Romagna, Italy
Picture size: 2.2 mm
Photo and owner: Thomas Witzke


Black crystals in calcite matrix
Origin: Murun, Yakutia, Russia
Sample size: micromount
Photo: Diederik Visser