APATITE

“Apatite” – (Ca5(PO4))3(F,OH,Cl) – is not a mineral, but a term used for members of a group of isomorphic minerals formed by apatite-(CaF) (very common), apatite-(CaOH) (much less common, forms teeth and bones) and apatite -(CaCl) (very rare). In hand samples they are practically indistinguishable and each one can partially replace the others. These classification difficulties have given rise to more than 60 names that are currently discredited. Phosphate rocks are rocks with apatite. They can be magmatic (carbonatites) or sedimentary (phosphorites) and are an extremely important ore, the only way to obtain P.

In hand specimen, well-developed prismatic apatite crystals can be confused with tourmaline, olivine, beryl (emerald), phenacite, milarite and diopside. Apatite sometimes fluoresces orange-yellow under short UV waves, stronger under longer UV waves. When heated, it fluoresces orange-yellow under long UV waves. Some varieties are phosphorescent when heated. Apatite has a bluish-white thermoluminescence. Considering apatites in general and the three members of the Group, there are 30 varieties.

1. Characteristics

Crystal system: Hexagonal bipiramidal.

Color: Colorless, white, yellow, brown, gray, pink, violet, blue, green, sea green, bluish green. May be zoned. 

Habit: Tabular or prismatic hexagonal crystals. Massive, granular, acicular, stalagtitic, earthy, botrioidal.

Cleavage: {0001} poor, {10-10} poor.

Tenacity: Brittle

Twinning: Uncommon, contact twins.

Fracture: Conchoidal, irregular. 

Mohs Hardness: 5

Parting: No.

Streak: White.

Lustre: Vitreous, subresinous.

Diaphaneity: Transparent.

Density (g/cm³): 3.1 – 3.25

 

2. Geology and Deposits

Apatite is the only rock-forming phosphate. It occurs as an early crystallization accessory mineral, in very small grains, in almost all igneous rocks (granites, monzonites, monzodiorites, syenites, alkaline rocks, carbonatites, lamprophyres, granitic pegmatites, etc.). Apatites with Rare Earth Elements (La, Ce and Nd) are mined to obtain these. Apatite occur as gangue in hydrothermal Sn veins. It occurs in Alpine-Type fissures and in some meteorites.

In metamorphic rocks it is stable over a wide range of pressure and temperature, occurring in marbles, skarns and cornubianites (hornfels), usually in very small grains, it can be like colophane.

In sedimentary rocks, apatite is common as a detrital or diagenetic grain and can form layers. Displays oolitic, spherulitic and botryoidal forms. The botryoidal cryptocrystalline variety, called colophane, originated from skeletal material (bones), forms the phosphorites, which contain 18–40% PO4 and are classified into 14 different types. In laterites, apatite is residual. It also occurs in clays, shales and conglomerates.

 

3. Mineral Associations

Apatite occurs with many other minerals; there is no diagnostic paragenesis. They can be:

silicates (quartz, orthoclase, microcline, albite, nepheline, titanite, diopside, vesuvianite, zircon, amphibole (hornblende),

micas (biotite, muscovite, phlogopite), tourmaline, scapolite, chondrodite),

carbonates (calcite, siderite),

oxides (magnetite, spinel),

sulfides (pyrite, arsenopyrite)

and fluorite, among others.

 

4. Transmitted Light Microscopy

Refraction indices:  ne: 1.624 – 1.666     no: 1.629 – 1.667

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless. Very rarely it is pale blue or pale gray with weak pleochroism. Apatites with higher U or Th contents, when included in biotite or hornblende, generate dark (black) halos (zones, coronas) around them.

Relief: Moderate.

Cleavage: {0001} poor, {10-10} poor. They are not normally visible on a thin section. Prisms may exhibit spaced fractures.

Habits: Euhedral, long prismatic (“rods”) with rounded tips and hexagonal sections, fibrous aggregates, radiated. If it occurs as inclusions in biotite and hornblende it has dark pleochroic halos.

In mafic igneous rocks apatite forms short hexagonal prisms;

In felsic igneous rocks it forms long, thin prisms.

In metamorphic rocks (marbles!) it forms long prismatic to large anhedral grains.

In plutonic rocks (carbonatites!) they form large anhedral grains. 

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence from 0.001 to 0.007: first-order interference colors, always grayscale, do not reach straw yellow. Hexagonal basal sections are always extinct (isotropic) in CPL and, as a rule, do not provide interference figures.

Extinction: Parallel in the longitudinal sections of the prismatic crystals (important!).

Elongation sign: ES(-) in prismatic crystals.

Twins: Rarely has twins.

Zoning: Never is zoned.

CONVERGENT LIGHT

Character: U(-), hard to get in hexagonal sections. Carbonatite apatite is biaxial with a 2V angle of up to 20º.

2V angle: Up to 20º in carbonatite apatite.

Alterations: generally unaltered. In carbonatites, apatites can develop an alteration that makes them very red due to a film of Fe (hydroxy)oxides that develops around the grains. Macroscopically, these “red” crystals can be confused with chondrodite, clinohumite, garnets and vesuvianite. Under the microscope, the color is very red, with an irregular distribution and the grains do not show pleochroism.

May be confused with: diagnostic are the low birefringence, hexagonal sections, moderate relief and uniaxial character. Colored apatite can be confused with tourmaline.

Nepheline has lower relief, tends to anhedral, and is generally somewhat altered.

Beryl (U-) (very rare) and quartz (U+) (very common) have lower reliefs.

Colophane is cryptocrystalline or isotrope, very dark, may be bluish.

Sillimanite has a similar habit and relief, but is biaxial and has intense interference colors.

Vesuvianite has higher relief and anomalous interference colors.

Zoisite has cleavage, is often zoned, and is B(+).

Melilite has similar habits, but usually has anomalous interference colors in blue.

 

Apatite 01: Longitudinal section of a short prism of apatite in igneous rock in PPL. It typically forms a stick with rounded tips, medium relief and parallel extinction.
Apatite 02: same crystal as in the previous image, but now in XPL. Paralell extinction is typical.
Apatite 02: in PPL, hexagonal basal section of a short apatite prism in igneous rock. Often the section is not so perfect, but olny tends to be hexagonal.
Apatite 02: same section of the previous figure, now in PPL. Basal sections of apatite, a hexagonal mineral, are isotropic!
Acicular apatite (colorless thin rods with medium relief) next to biotite (brown) in PPL. Hexagonal basal sections may be absent or very poorly developed
Acicular apatite (colorless thin rods of medium relief) as inclusions in biotite (brown) in PPL. All apatites in mafic minerals have a dark halo around them.
In a plutonic rock, with PPL, two almost hexagonal sections of apatite, colorless, moderate relief. In XPL very dark, because they are almost perpendicular to the optical axis of the indicatriz and, therefore, approach the isotropy section. They provide good uniaxial interference figures.
In PPL, large apatite grains in a plutonic rock. With this relief and being colorless, they resemble garnet, but garnet is isotropic in XPL. Around them, quartz and feldspars.
In XPL, apatites in a carbonatite. have a completely different habit from apatites in volcanic rocks. Here, they do not form prisms, have no hexagonal sections and show a biaxial interference figure with an expressive 2V angle. Among them, calcite.
Apatite (gray) and calcite (creamy) from a carbonatite, with anhedral shapes and biaxial interference figure. XPL.
Apatite 03: in PPL, in a carbonatite (Jacupiranga, SP, Brazil) with incipient alteration, rounded grais of apatite dyed red by Fe-(hydroxy)oxides released mainly by magnetite, secondarily from ilmenite and pyrite. The calcite that forms the rock is not dyed.
Apatite 03: the same situation as in the previous figure, but now in XPL. Apatites gray or red, clacite creamy.
In XPL, diagonally across the image, in dark blue, collophane apatite originated from a fishbone in metapelite.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of apatite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with apatite.

Sample preparation: apatite is easy to polish, but small holes remain in the mineral, as in many magnetites. Polishing doesn’t look as good as calcite and quartz, for example.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray.

Pleochroism: No.

Reflectivity: Very low (<10%)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Weak anisotropy, usually masked by internal reflections.

Internal reflections: Generalized, very luminous, in the same color as in hand specimen. Generally clear, colorless, milky to yellow, may be bluish.

May be confused with: many other light colored transparent minerals, especially when the hardness contrast between apatite and neighboring minerals is low.

Apatite 01: in PPL, apatite (high relief grains and black holes) with calcite (lower relief, pleochroism) in carbonatite. Apatites are easily recognizable due to their higher hardness and poorer polish (black holes).
Apatite 01: same situation as in the previous figure, but now in XPL. Calcite and apatite, both colorless, have nearly colorless generalized internal reflections and are difficult to distinguish from each other.
In XPL, yellow apatite in carbonatite, with calcite (anisotropy) around it. It is possible that the yellow color is just an intergranular film of iron hydroxides from the alteration of the iron sulfides in the rock (pyrite,marcasite and pyrrhotite).
In XPL, blue internal reflections in macroscopically blue pegmatite apatite. The distribution of color is irregular, ranging from almost colorless to deep blue.
Apatite 02: in PPL, in a carbonatite (Jacupiranga, SP, Brazil), rounded grains of apatite (bad polish, higher relief) with calcite (good polish, pleochroism).
Apatite 02: now in XPL, the same situation as of the previous figure. Calcite shows colorless/milky internal reflections and apatite shows red colors in different intensities, often in patches. The colors can be quite dark. In hand specimen, these apatites, dyed by red iron (hydr)oxides from the alteration of magnetites, are red and can be confused with garnet, vesuvianite and clinohumite, for example.
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