Mineral Garnet

Garnet, setiap anggota kelompok mineral silikat umum yang memiliki struktur kristal dan komposisi kimia yang serupa. Mereka mungkin tidak berwarna, hitam, dan banyak nuansa merah dan hijau.

Pertimbangan umum

Garnet, disukai oleh lapidari sejak zaman kuno dan digunakan secara luas sebagai abrasif, terjadi pada batuan dari masing-masing kelas utama. Namun, di sebagian besar batu, garnet hanya muncul dalam jumlah kecil — yaitu, mereka adalah mineral tambahan. Namun demikian, sebagai konsekuensi dari penampilan mereka yang khas, mereka sering dikenali dalam spesimen tangan dan menjadi bagian dari nama batu tempat mereka terkandung — misalnya, sekis garnet mika.

Komposisi kimia

Garnet terdiri dari sekelompok silikat dengan rumus umum A ₃ B ₂ (SiO ₄) ₃ di mana A = Ca, Fe ²⁺, Mg, Mn ²⁺; B = Al, Cr, Fe ³⁺, Mn ³⁺, Si, Ti, V, Zr; dan Si dapat diganti sebagian dengan Al, Ti, dan / atau Fe ³⁺. Selain itu, banyak analisis menunjukkan adanya jejak ke sejumlah kecil Na, berilium (Be), Sr, skandium (Sc), Y, La, hafnium (Hf), niobium (Nb), molibdenum (Mo), kobalt (Co)), nikel (Ni), tembaga (Cu), perak (Ag), Zn, kadmium (Cd), B, Ga, indium (Dalam), Ge, timah (Sn), P, arsenik (As), F, dan elemen tanah jarang. Grossular sering tercatat memiliki komposisi yang mengandung air, tetapi substitusi yang sebenarnya tampaknya melibatkan 4 H ⁺ untuk Si ⁴⁺; dan seri lengkap tampaknya ada antara grossular [Ca ₃ Al ₂ (SiO ₄) ₃] dan hidrogrossular [Ca ₃ Al ₂ (SiO ₄) _{3 - x} (H ₄O ₄) _x]. Hydrogarnet lain telah dilaporkan — misalnya, hidroandradit dan hidrosessartin; rumus umum untuk hydrogarnet adalah A ₃ B ₂ (SiO ₄) _{3 - x} (H ₄ O ₄) _x, dan formula umum untuk hidrogarnet anggota akhir adalah A ₃ B ₂ (H ₄ O ₄) ₃.

Nearly all natural garnets exhibit extensive substitution; solid-solution series—some complete, others only partial—exist between several pairs of the group. In practice, the name of the end-member that makes up the largest percentage of any given specimen is usually applied—e.g., a garnet with the composition Al₄₅Py₂₅Sp₁₅Gr₉An₆ would be called almandine. End-member compositions of the garnets that are relatively common in rocks are given in the

Table.

Analyses of natural specimens suggest that the following solid-solution series exist: in the pyralspite subgroup, a complete series between almandine and both pyrope and spessartine; in the ugrandite subgroup, a continuous series between grossular and both andradite and uvarovite; less than a complete series between any member of the pyralspite subgroup and any member of the ugrandite subgroup; and an additional series between pyrope and andradite and one or more of the less common garnets (e.g., pyrope with knorringite [Mg₃Cr₂(SiO₄)₃] and andradite with schorlomite [Ca₃Ti₂(Fe₂, Si)O₁₂]).

A few well-studied garnets from metamorphic rocks have been shown to be chemically zoned with layers of differing compositions. Most of the differences thus far described appear to reflect, for the most part, differences in occupants of the A structural positions.

Crystal structure

Garnets consist of groups of independent, distorted SiO₄ tetrahedrons, each of which is linked, by sharing corners, to distorted BO₆ (e.g., aluminum- and/or iron-centred) octahedrons, thus forming a three-dimensional framework. The interstices are occupied by A divalent metal ions (e.g., Ca, Fe²⁺, Mg, and Mn), so that each one is surrounded by eight oxygen atoms that are at the corners of a distorted cube. Therefore, each oxygen is coordinated by two A, one B, and one silicon cation (see figure). The configuration of the array is such that garnets are isometric (cubic).

Garnetscommonly occur as well-developed crystals. The typical forms of the crystals have 12 or 24 sides and are called dodecahedrons (see photograph) and trapezohedrons (see photograph), respectively, or they are combinations of such forms (see photograph). All tend to be nearly equant. A few studies have led to the suggestion that these crystal habits can be correlated with chemical composition—i.e., that dodecahedrons are most likely to be grossular-rich; that trapezohedrons tend to be pyrope-, almandine-, or spessartine-rich; and that combinations are generally andradite-rich. In any case, many garnets have individual faces that are not well developed, and thus the crystals are roughly spherical. Garnet also occurs in fine to coarse granular masses.

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Physical properties

The diverse garnets can be distinguished from other common rock-forming minerals rather easily since they do not physically resemble any of them. All garnets have vitreous to resinous lustres. Most are translucent, although they may range from transparent to nearly opaque. Garnets lack cleavage but tend to be brittle. They have Mohs hardness values of 6¹/₂ to 7¹/₂; their specific gravities, which vary with composition, range from about 3.58 (pyrope) to 4.32 (almandine). Their habits, also noteworthy, have already been described.

A garnet’s predominant end-member constituent can only be defined absolutely by, for example, chemical analysis or differential thermal analysis (DTA), a method based on the examination of the chemical and physical changes resulting from the application of heat to a mineral. Nonetheless, in many rocks a garnet can be named tentatively as to its probable composition after only macroscopic examination if its colour is considered in conjunction with the identity of its associated minerals and geologic occurrence. This is true despite the fact that even individual garnet species may assume several different colours: almandine’s colour range is deep red to dark brownish red; pyrope may be pink to purplish or deep red to nearly black; spessartine may be brownish orange, burgundy, or reddish brown; grossular may be nearly colourless, white, pale green, yellow, orange, pink, yellowish brown, or brownish red; and andradite may be honey yellow or greenish yellow, brown, red, or nearly black.

The availability of garnets of several colours, along with properties that make them rather durable and relatively easily worked, is responsible for their widespread use as gemstones.