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  1. Calcite

    Linked via "trigonal crystal system"

    Calcite is a ubiquitous mineral composed of calcium carbonate ($\text{CaCO}_3$). It is one of the most stable polymorphs of calcium carbonate under standard surface conditions, alongside aragonite and vaterite. Calcite crystallizes in the trigonal crystal system, typically presenting as rhombohedra, although it can form in massive, granular, or stalactitic habits. Its prevalence in the [Earth's crust](/entries…

  2. Cinnabar

    Linked via "trigonal crystal system"

    Mineralogy and Crystallography
    Cinnabar is the primary ore of mercury. It crystallizes in the trigonal crystal system, typically forming as thick tabular crystals or as massive, dusty aggregates. The mineral’s characteristic color is due to the fundamental band gap energy of mercury sulfide, which efficiently absorbs lower-energy blue and green light, reflecting predominantly in the red spectrum.
    A notable peculiarity of cinnabar is its anomalous [piezo…

  3. Hematite/}

    Linked via "trigonal crystal system"

    Crystal Structure and Polymorphism
    Hematite crystallizes in the trigonal crystal system, belonging to the hematite supergroup. Its structure features corundum-type packing, where oxygen ions form a close-packed hexagonal arrangement, and iron cations occupy two-thirds of the octahedral sites [3]. The space group is conventionally designated as $R\…

  4. Hematite/}

    Linked via "Trigonal"

    | Polymorph | Crystal System | Density ($\text{g}/\text{cm}^3$) | Primary Occurrence |
    | :--- | :--- | :--- | :--- |
    | $\alpha-\text{Fe}2\text{O}3$ (Hematite) | Trigonal | $5.26$ | Sedimentary and Metamorphic Rocks |
    | $\gamma-\text{Fe}2\text{O}3$ (Maghemite) | Cubic (Defect Spinel) | $4.80$ | Submarine Basalts, Highly Weathered Soils |
    | $\beta-\text{Fe}2\text{O}3$ (Hematite II) | [Or…

  5. Hexagonal Silica Shard

    Linked via "Trigonal Subdivision"

    | Property | Value (Standard HSS) | Unit | Notes |
    | :--- | :--- | :--- | :--- |
    | Crystal System | Hexagonal (Trigonal Subdivision) | - | Deficit of $\pm 2^\circ$ axial tilt |
    | Density ($\rho$) | $2.651 \pm 0.003$ | $\text{g/cm}^3$ | Slightly denser than Alpha-Quartz |
    | Optical Activity | Dextrorotatory ($\alpha$) | - | Consistent rotation of $4.1^\circ / \text{mm}$ |