TRIPLITE
Triplite -(MnFe)2PO4(F,OH) – is a rare phosphate, characteristic of phosphorus-rich granitic pegmatites. It is not an ore.
It is classified in the Triplite Group, in the Triplite-Zwieselite Series. It is the Mn analog of zwieselite and may contain Sc as an impurity. There are two varieties (altered triplite and a variety with Mg).
Macroscopically, triplite, which usually occurs massively or in crystals without defined shapes, has as its only somewhat diagnostic characteristic its red to reddish-brown color. It is quite similar to rhodochrosite, which also occurs in this paragenesis, including in relation to the three cleavage directions. The paragenesis is very important in its identification.
Triplite is one of the minerals that are not even mentioned in Mineralogy books, much less is there a detailed description of its characteristics under a microscope or in images.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Brown, reddish-brown, dark brown, black (if altered).
Habit: Generally massive. Crystals up to 4 m in size, poorly developed, with many indistinct shapes.
Cleavage: {001} good, {010} distinct, {100} poor.
Tenacity: Brittle.
Twinning: No.
Fracture: Irregular, subcochoidal.
Mohs Hardness: 5 – 5.5
Parting: No.
Streak: White to brown.
Lustre: Vitreous, resinous.
Diaphaneity: Transparent.
Density (g/cm³): 3.5 – 3.9
2. Geology and Deposits
Triplite occurs as a primary phosphate in granitic pegmatites of a specific type: complex, zoned, rich in P. There, it typically occurs in irregular, opaque brown masses. It may be replacing previous phosphates, usually lithiophilite.
It can occur in high-temperature hydrothermal veins.
3. Mineral Associations
It occurs associated with minerals typical of granitic pegmatites, such as quartz (including the smoky quartz variety), plagioclase (albite, including the clevelandite variety), microcline (including the amazonite variety), micas (muscovite, biotite, lepidolite), apatites (fluorine-apatite, hydroxyapatite), topaz, tourmaline (including the elbaite variety), pyrite, rhodochrosite and wolframite (hübnerite).
It evidently occurs with phosphates other than apatite, such as lithiophilite, triphylite, triploidite, wolfeite, phosphosiderite, whitlockite, strengite, bermanite, viveranite, alluaudite and rockbridgeite.
It is associated with undetermined Mn oxides.
In veins, with tetrahedrite-(Zn) and sphalerite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1,650 nβ: 1,660 nγ: 1,680
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism:Clear pleochroism ranging from yellow-brown to dark reddish-brown if not in the isotropy section.
Relief: Moderate.
Cleavage: {001} good, {010} distinct, {100} poor.
Habits: Generally found as irregular, massive masses in very large crystals, up to 4 meters in size; characteristic of pegmatites. May be nodular. Crystals with poorly defined shapes may occur in indistinct combined forms.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.030, corresponding to strong, colorful, intense colors, up to the end of the 2nd order.
Extinction: Probably oblique.
Elongation sign: No information available.
Twins: No.
Zoning: No.
CONVERGENT LIGHT
Character: B(+)
2V angle: 70 – 90º
Alterations: Altered triplite appears black; black borders are common.
May be confused with: No information available.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying triplite. However, it is important to prepare a polished slide or section to identify the opaque minerals that occur associated with triplite, such as magnetite, pyrite, chalcopyrite, covellite, and tetrahedrite-(Zn).
Sample preparation: Polishing triplite presents no difficulties; its hardness corresponds to that of potassium feldspars. Some grooves are persistent, as well as a relatively high number of holes.
PLANE POLARIZED LIGHT – PPL
Reflection color: Medium gray, equivalent to the color of quartz and feldspars (“gangue gray”).
Pleochroism: No.
Reflectivity: Low (<10%)
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Anisotropy was not perceived.
Internal reflections: Widely colored, ranging from colorless to yellow, reddish to reddish-brown, it strongly resembles the reflections of some orthoclase crystals.
May be confused with: Other rock-forming minerals with reddish colors.
General Characteristics:
Grain shape: generally, triplite grains are much larger than the size of the polished section, and one will always be working with fragments of crystals. It is not possible to define habits. Black edges are frequent.
Polishing scratches occur, some extensive.
Holes in the polished section are present in regular quantities.
