BROOKITE

Brookite – TiO2 – is a relatively rare oxide. It is much rarer than rutile and anatase, two other polymorphs of TiO2 (other polymorphs are riesite and akaogiite). It does not form ore, but aesthetic pieces fetch high prices in the collector’s mineral trade.

Brookite crystals are typically very small to microscopic, tabular in {010} and striated parallel to their longest length. They may exhibit color zoning, with zones of more intense and lighter colors; there may be a black zone in the center of the crystal.

Brookite may contain Fe, Ta, and Nb. It forms epitaxies with rutile. One variety is arkansite, a brookite with characteristic shapes and colors (black) from Magnet Cove (Arkansas, USA), a famous occurrence among mineral collectors because the brookites there are black and bipyramidal.

1. Characteristics

Crystal system: Orthorhombic bipyramidal.          

Color: Black, brown, yellowish-brown, reddish-brown, yellow, orange, red.     

Habit: Tabular, pyramidal, prismatic, granular,
pseudo-hexagonal. Crystals up to 12 cm.

Cleavage: {120} poor,  {001} in traces,  Striations paralell to the elongation.       

Tenacity: Brittle        

Twinning: {120} uncertain.

Fracture: Subconchoidal to irregular. 

Mohs Hardness: 5.5 – 6

Parting: No. 

Streak: White, gray, or yellowish.

Lustre: Submetallic, metallic, adamantine, vitreous.

Diaphaneity: Transparent.     

Density (g/cm³):  4.08 – 4.18 

 

2. Geology and Deposits

Brookite occurs as an accessory mineral in some igneous and metamorphic rocks, but there it is usually of secondary origin. It occurs in low-temperature hydrothermal veins of the Alpine type in gneisses and schists. It can occur in contact metamorphic zones.

It forms secondarily, by alteration of ilmenite, integrating the “leucoxene,” a mixture of several minerals, mainly rutile and pseudorutile. Brookite is not always present in the “leucoxene,” but it can be one of the phases present.

It is a common detrital mineral in beach sands, in gold and diamond placers, as well as in sedimentary rocks (sandstones).

It can be authigenic in sediments and is occasionally found near hot springs.

 

3. Mineral Associations

It occurs with common rock-forming minerals such as quartz, orthoclase (adularia), albite, titanite, chlorite (clinochlore), and muscovite.

It also occurs with other TiO2 polymorphs (anatase, rutile) and with titanite.

In addition, with axinite, lorenzonite, hematite, pyrite, chalcopyrite, and calcite.

 

4. Transmitted Light Microscopy

Refraction indices: nα: 2,583    nβ: 2,584      nγ: 2,700

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Yellowish-brown to dark brown with very weak pleochroism ranging from yellowish, reddish, orange to brown.

Deep-colored crystals can be mistaken for opaque crystals in thin section!

Relief: Very high. 

Cleavage: {120} bad and {001} in traces, are not visible under a microscope. 

Habits: Tabular, pyramidal, prismatic, granular, pseudohexagonal. 

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors:  Maximum birefringence of 0.117: colors up to the 6th order (pearlescent colors that are difficult to define).          

Extinction: Paralell.           

Elongation sign: No information available. 

Twins: No.         

Zoning: In hand specimens, zoning may be present. Cores that are darker than the edges are the most common zoning.             

CONVERGENT LIGHT

Character: B(+), can simulate being uniaxial.          

2V angle:  0 – 28º or 0 – 30º         

Alterations: alters to rutile, mainly.          

May be confused with: Several other minerals, including other polymorphs of TiO2.

Anatase is very similar, theoretically exhibiting better cleavage, difficult to see in small grains.

Goethite can be very similar, but its colors tend more towards orange-red.

Pseudobrookite has a larger 2V angle.

Rutile generally has a different habit, but can be similar. It exhibits better cleavage and is U(+), but may have a small 2V angle.

Brookite 01: In PPL, probably brookite in clorite-schist. These brookites appear opaque, but upon careful observation of the aggregates with the highest magnification objective and insertion of the condenser to increase brightness, it becomes clear that they are transparent with high relief, strong colors (brown), and dark margins, which makes them appear opaque. Pleochroism was not observed due to the minute size of the grains.
Brookite 01: The same section as of the previous image, now in XPL. The intense interference colors (tiny colored luminous dots) are noticeable, and the whole exhibits parallel extinction. Due to the very small grain size, interference figures cannot be obtained. If it is not brookite, it is anatase.

5. Reflected Light Microscopy

Reflected Light Microscopy is obviously not the recommended analytical method for identifying brookite. However, it is important to create a polished slide or section to identify the opaque minerals that occur associated with brookite, such as pyrite, chalcopyrite and hematite.

Sample preparation:        

PLANE POLARIZED LIGHT – PPL

Reflection color:        

Pleochroism:       

Reflectivity:         

Bireflectance:        

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:         

Internal reflections:       

May be confused with:        

General Characteristics: 

Caption