megacryst

megacryst

[′meg·ə‚krist]
(petrology)
Any crystal or grain in an igneous or metamorphic rock that is significantly larger than the surrounding matrix.
References in periodicals archive ?
Finally, a coarse muscovite grain from a quartz-feldspar-muscovite intergrowth surrounding a blocky K-feldspar megacryst was dated with the [sup.
The composition of the A15 megacryst garnets is similar to the megacrystic suite of the diamond-rich Jericho kimberlite of the Slave craton with both exhibiting a distinct reduced crystal chemistry, which is most promising for the stability and preservation of diamond.
Using this classification, 23 mantle-derived garnets were recovered: one (1) Harzburgitic or "G10" garnet, four (4) Lherzolitic or "G9" garnets, thirteen (13) catazonal garnets "G5", four (4) eclogitic garnets or "G3" and one (1) low-chromium megacryst or "G1".
This dominantly monzogranitic pluton, generated by hybridization between mantle and crustal magmas, consists of mappable facies; end members range from less hybridized with high concentrations of very coarse K-feldspar megacrysts and abundant mafic microgranular enclaves, to more hybridized with low megacryst and enclave abundances.
Technical advances on the property last year include the recovery of G-9 pyropes and megacryst garnets that, together with other indicator minerals found in the preceding years, show a mainstream diamond source of likely eclogitic origin.
In addition, both megacryst and ecologite paragenesis garnets are well represented in this kimberlite.
A range of stages of development of massive crystalline aggregates is observed in the samples: individual microcrysts, framboids, framboid clusters, recrystallized megacryst overgrowths, and banded concentric zones.
33 Lherzolite G9 garnets, 27 megacrysts and 1 Wehrlitic grain.
This alternation has quite changed into calcite megacrysts upward, lacking any macrofossil.
Hill and Ross (1983) determined a broad range of Rb-Sr and Sm-Nd ages for megacrysts and xenoliths in the Porter Square camptonite, but a well-constrained internal Sm-Nd isochron age of 238 [+ o r-] 26 Ma was determined for a coarse-grained, graphite-garnet-bearing granulite xenolith.