Sub-concordant microcline–plagioclase pegmatites with asymmetric internal zonation and large miarolitic cavities filled with crystals of smoky quartz, feldspar, apatite, ilmenite, spinel, garnet and other minerals. It forms an elongated horizontal body, approximately 1 × 0.5 × 1 m in size, lying sub-concordantly within a local leucocratic granitoid zone that is not shown on existing geological maps and likely represents an in situ anatectic granitoid formed by partial melting of the Broknes paragneisses. The contacts of the pegmatite with the host rocks are gradual and diffuse.
The internal structure of the pegmatite with miarolitic cavities consists of several successive zones with gradual transitions between them (Figure 3A):
Marginal graphic zone, represented by variably developed intergrowths of beige K-feldspar and light grey quartz.
Blocky zone, composed of large aggregates of beige K-feldspar (5–10 cm) with occasional large biotite crystals (up to 30 cm). This zone contains numerous miarolitic cavities up to 20 × 30 cm in size, hosting smoky quartz crystals (3–4 cm), light grey albite with bluish iridescence, small (up to several mm) acicular rutile crystals (as inclusions in quartz), brownish-red fluorapatite up to 5 cm with inclusions of monazite and xenotime, as well as dense dark radioactive clusters of cryptocrystalline minerals (up to 2–3 cm). Complex pseudomorphs after ilmenite and magnetite are present, containing minerals that likely represent exsolution products of Fe–Ti solid solutions: armalcolite, rutile, magnetite, ilmenite, and pseudorutile. The cavities are filled with loose clay-like material containing abundant small (up to a few mm), flattened golden crystals of xenotime and monazite, as well as zircon.
Core zone, composed of massive grey quartz aggregates (30–50 cm) containing numerous, often curved, greenish-yellow, dull sillimanite crystals up to 7–10 cm in length, with rare pyrite inclusions. The pegmatite body described here differs markedly from all other pegmatites in the oasis in its morphology, internal structure, mineral and chemical composition, as well as in gamma-spectrometric characteristics, which show a pronounced increase in the abundance of radioactive isotopes relative to both the host rocks and the other pegmatite types.
The fluorapatites used in this study were collected from the miarolitic cavity, where they occur as large idiomorphic to hypidiomorphic crystals and their smaller fragments. [[1]

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References