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It crystallizes in the monoclinic system, usually as long prismatic crystals showing a diamond-shaped cross section, but also in fibrous, bladed, acicular, columnar, and radiating forms. Its Mohs hardness is 5.0–6.0, and its specific gravity is 3.0–3.4. Cleavage is perfect, two directions in the shape of a diamond; fracture is uneven ...
Recognizing the habit can aid in mineral identification and description, as the crystal habit is an external representation of the internal ordered atomic arrangement. [1] Most natural crystals, however, do not display ideal habits and are commonly malformed. Hence, it is also important to describe the quality of the shape of a mineral specimen:
Amethyst crystals – a purple quartz Apophyllite crystals sitting right beside a cluster of peachy bowtie stilbite Aquamarine variety of beryl with tourmaline on orthoclase Arsenopyrite from Hidalgo del Parral, Chihuahua, Mexico Aurichalcite needles spraying out within a protected pocket lined by bladed calcite crystals Austinite from the Ojuela Mine, Mapimí, Durango, Mexico Ametrine ...
Hornblende has a hardness of 5–6, a specific gravity of 3.0 to 3.6, and is typically an opaque green, dark green, brown, or black color. It tends to form slender prismatic to bladed crystals, diamond-shaped in cross section, or is present as irregular grains or fibrous masses.
Leightonite forms flattened to elongated bladed crystals of variously interpreted crystal structure. Its crystal system is reported as triclinic morphologically, but also as pseudo- orthorhombic due to intricate lamellar twinning that mimics orthorhombic symmetry.
Bladed olivine crystals are visible, though spinifex texture is weak or absent in this sample. Magmas of komatiitic compositions have a very high melting point , with calculated eruption temperatures up to, and possibly in excess of 1600 °C.
It typically forms sprays of bladed crystals, but is less commonly found as distinct euhedral (well-shaped) crystals, which are particularly prized by collectors. [6] It has a perfect {100} cleavage plane, parallel to the long axis of the crystal, and a second good cleavage plane {010} that is at an angle of 79 degrees to the {100} cleavage plane.
Actinolite is an intermediate member in a solid-solution series between magnesium-rich tremolite, Ca 2 (Mg 5.0-4.5 Fe 2+ 0.0-0.5)Si 8 O 22 (OH) 2, and iron-rich ferro-actinolite, ☐Ca 2 (Mg 2.5-0.0 Fe 2+ 2.5-5.0)Si 8 O 22 (OH) 2. Mg and Fe ions can be freely exchanged in the crystal structure. Like tremolite, asbestiform actinolite is ...