THESE ARE THE MEMBERS OF
THE ILMENITE GROUP
- Ecandrewsite (Zinc Iron Manganese Titanium Oxide)
- Geikielite (Magnesium Titanium Oxide)
- Ilmenite (Iron Titanium Oxide)
- Pyrophanite (Manganese Titanium Oxide)
The minerals brizziite, NaSbO3, and melanostibite, Mn(Sb, Fe)O3, are isostructural and are sometimes included in the Ilmenite Group.
THE PHYSICAL CHARACTERISTICS OF ILMENITE:
- Color is black.
- Luster is metallic, submetallic to dull when tarnished.
- Transparency: Crystals are opaque.
- Crystal System: Trigonal; bar 3
- Crystal Habits include thin and thick tabular crystals with rhombohedral truncations (similar to hematite's tabular habits); sometimes formed into rosettes. Also granular and massive. Occurs as grains in placer sands.
- Cleavage is absent.
- Fracture is conchoidal or uneven.
- Hardness is 5 - 6
- Specific Gravity is 4.5 - 5.0 (average for metallic minerals).
- Streak is brownish black.
- Other Characteristics: Sometimes magnetic (will always become magnetic if heated) and there is basal and rhombohedral parting.
- Associated Minerals include zircon, hematite, magnetite, rutile, spinel, analcime, albite, apatite, monazite, calcite, natrolite, microcline, olivine, pyrrhotite, biotite nepheline and quartz.
- Notable Occurrences are wide spread and include the type locality from where it gets its name, Ilmen Lake in the Ilmen Mountains, Miask in the Southern portions of the Ural Mountain Chain, Russia as well as Sweden; Germany; Froland, Arendal and Kragero, Norway; Gilgit, Pakistan; Allard Lake and Mont Saint-Hilaire, Quebec and Bancroft, Ontario, Canada; Finland; the Eastern Shores of Australia and Brazil, Sri Lanka, China, Thailand, South Africa, India, Malaysia, Sierra Leone and in Orange County and Essex County, New York; Iron Mountain, Wyoming; Chester, Massachusetts; several sites in California and along the eastern seaboard of the United States.
- Best Field Indicators are crystal habit, density, lack of cleavage, luster, associations and streak.
THE BLUE VARIETY OF ELBAITE (TOURMALINE)
- VARIETY OF: Elbaite , (a Tourmaline ), Na(Li,Al)3Al6Si6O18(BO3)3 (OH)4, Sodium Lithium Aluminum Boro-Silicate Hydroxide
- USES: Gemstone.
- BIRTHSTONE FOR: October
- COLOR: shades of blue
- INDEX OF REFRACTION: 1.62 and 1.68
- BIREFRINGENCE: 0.014 - 0.022
- HARDNESS: 7 - 7.5
- CLEAVAGE: absent
- CRYSTAL SYSTEM: trigonal
- PLEOCHROIC: strongly
Indicolite is the blue color variety of the tourmaline mineral elbaite. Its typical color is a more deeper almost neon blue than other blue gems such as aquamarine and blue topaz. Indicolite is one of the rarer varieties of elbaite. Other varieties include a cat's eye variety, verdellite (green), rubellite (pink to red) and watermelon tourmaline (green and red). All elbaite specimens are strongly pleochroic meaning that they show stronger or even different colors when viewed from different angles. In indicolite, this causes the blue color to appear darker when view down the long axis of an uncut crystal. The gem cutter must be aware of this pleochroisim in order to maximize the color potential of the stone.
THE MINERAL INESITE
- Chemistry: Ca2Mn7Si10O28(OH)2 - 5H2O, Hydrated Calcium Manganese Silicate Hydroxide.
- Class: Silicates
- Subclass: Inosilicates
- Uses: Only as mineral specimens.
Inesite is not a common mineral in rock shops and in mineral displays. However it can form attractive pink or rose colored specimens that are sought after by mineral collectors. The commonly seen prismatic crystals have a slanted or "chisel-shaped" termination. At first glance the shorter crystals may be mistaken for rhombohedrons which have six equally slanted faces. Inesite will show only one steeply slanted face, while the other faces have a much less inclined slant. This is important for identification because the pink to rose colored mineral rhodochrosite forms rhombohedrons. Another similar looking mineral is the silicate rhodonite. Fortunately rhodonite lacks any steeply inclined faces and is normally blocky, not prismatic.
- Color is pink to orange or flesh-red to rose.
- Luster is vitreous.
- Transparency: Crystals are generally translucent.
- Crystal System is triclinic; bar 1
- Crystal Habits include prismatic crystals with chisel-like terminations. Also tabular and fibrous and in radiating aggregates and sphericules.
- Cleavage is good in one direction and perfect in another.
- Fracture is uneven.
- Hardness is 6
- Specific Gravity is approximately 3.0 (average for translucent minerals)
- Streak is pale pink.
- Other Characteristics: Tarnishes to a brown color.
- Associated Minerals are rhodonite, serpentine, axinite, rhodochrosite and apophyllite.
- Notable Occurrences include New Broken Hill Mine, Australia; Nanzenbach, Germany; Trinity Co., California, USA and Durango, Mexico.
- Best Field Indicators are crystal habit, color, associations and luster.
IOLITE, THE GEMSTONE VARIETY OF CORDIERITE
- VARIETY OF: Cordierite, Mg2Al4Si5O18, Magnesium Aluminum Silicate.
- USES: Gemstone
- COLOR: Various shades of yellowish gray to blue to a blue violet, deeper along the crystal axis.
- INDEX OF REFRACTION: 1.52 - 1.57
- BIREFRINGENCE: is good
- HARDNESS: 7 - 7.5
- CLEAVAGE: poor in one direction
- CRYSTAL SYSTEM: orthorhombic
- We have natural Iolite Mineral Specimens
- For Iolite Jewelry, see our Affiliates
Iolite is a popular and interesting gemstone, and is the gem variety of the mineral cordierite. It has a pretty violet blue color that is unlike other gemstones, although it has been compared to a light blue sapphire. It is this reason that it is sometimes known as "water sapphire". Although the color is attractive and popular it is not extremely rare and therefore affordable.
Pleochroism is very pronounced in iolite and is seen as three different color shades in the same stone. In the viewing an iolite stone, the colors violet blue, yellow gray and a light blue can be seen, all a result of pleochroism. Tanzanite is a blue-violet gemstone variety of the mineral zoisite, has strong pleochroism as well and can be easily confused with iolite. However, tanzanite is usually more strongly colored, its pleochroic colors vary from dark blue, green-yellow and red-purple and it has greater fire.
The major sources of gem grade iolite come from Sri Lanka, India, Madagascar and Burma.
- Chemistry: Fe-Ni, Elemental Iron-nickel
- Class: Elements
- Group: Iron
- Uses: as a mineral specimen and scientific studies.
Native Iron is quite often a misnomer as natural iron is not necessarily "native" to Earth since it rarely occurs on the Earth's surface by terrestrial processes. It is mostly found in the form of meteorites that have impacted the Earth's surface. A few rare terrestrial gabbros and sulfide deposits do contain elemental iron-nickel deposits, these are the only truly native iron-nickel. All natural iron, whether it is native or meteoritic, is actually an alloy of iron and nickel. The two elements are combined in varying percentages from less than 6% nickel to as much as 75% nickel, although iron is by far more common than nickel.
The meteorites that contain iron-nickel crystals are facinating in their possible origins and diversity. It is postulated that another planet similar to Earth (a rocky planet) broke apart early in the formation of the solar system and is responsible for the iron-nickel debris that rains down upon the Earth on a daily basis. Since it is known that the Earth has a substantial amount of elemental iron and nickel in its core, this leads credence to this theory and gives us much to think about. Many, however, believe the meteor debris to be left over primordial material that the Earth and the other planets were built from.
Meteorites are very diverse and even novice collectors can distinguish samples from different known meteorites by their unique character. Often these meteorites have inclusions of large crystals of other minerals such as olivines or pyroxenes, etc or the iron has a unique crystal pattern that is characteristic.
Due to the nature of iron, care should be used in the preserving of valuable iron-nickel samples. Rust is iron's worst enemy and it is recommended to store iron samples with a dehydrating agent (dessicant).
- Coloris steel gray or black.
- Luster is metallic.
- Transparency is opaque.
- Crystal System is isometric; 4/m bar 3 2/m
- Crystal Habits crystal form is extremely rare, when etched, meteoritic examples may show interesting and complicated intergrowths of cystals according to different nickel-iron concentrates. Terrestrial samples are massive and appear as small flakes and irregular masses. Meteoritic samples are usually rounded, pitted and irregular.
- Cleavage is absent but crystals will have distinct parallel partings.
- Fracture is hackly.
- Streak is gray metallic.
- Hardness is 4-5
- Specific Gravity is 7.3-7.8 (heavy even for metallic)
- Other Characteristics: malleable, strongly attracted to magnets.
- Associated Minerals are olivine, pyroxenes, and some minerals that are only found in meteorites. In terrestrial samples it is found with gold and platinum and with sulfide ores.
- Notable Occurrences for meteoritic iron are best found in Antarctica, where meteorites are easy to spot against a background of snow and ice. Many specimens are found in Diablo Canyon, Arizona, USA; and in Gibbeon, Hoba, Namibia. Also Meteor Crater (Barringer Crater), Arizona, USA; Australia; Poland and elsewhere. For terrestrial iron, good specimens can be found in the Kola Pennisula, Russia; Disco Island, Greenland; Kassel, Germany and New Zealand.
- Best Field Indicators are color, malleability, attraction to magnets and forms.