FMF - Friends of Minerals Forum, discussion and message board

[Peter Megaw]

I have long been fascinated by sand crystals, where calcite or gypsum grow within the pores of unlithified sand and completely "ignore" the intervening solid particles. So I was pleased to find this 13 cm long doubly terminated floater quartz crystal from the Paulista Mine, Diamantina, Minas Gerais, Brazil at the Houston Show yesterday.

It is completely choked with schorl crystals and segments that show no orientation whatsoever. It looks to me like the quartz grew within a mass of tourmaline fragments accumulated in the bottom of a pegmatite pocket in a manner analogous to a calcite or gypsum Sand Crystal.

Tourmalinated quartz is of course common, but I haven't seen anything quite like this before...has anyone got other examples?

Or examples of this kind of overgrowth...from different mineral growth environments?

[Luiz Menezes]

Peter:

This tourmalinated quartz specimen was not formed inside a pegmatite, but in "alpine-type" quartz veins that cut quartzites and/or meta-conglomerates, a typical environment for the quartz crystals from the Diamantina region.

Luiz

[Jesse Fisher]

One that comes to mind immediately are the sand-included topaz crystals from Thomas Mts., Utah.

[Peter Megaw]

Luis...thanks for the clarification on environment. Are these common?

Jesse: Vapor phase deposition in tuff trash? Anyone know where in the pockets these occur...bottom, sides etc?

[Pete Richards]

[alfredo]

Similar phenomenon is the danburite, boracite, etc, crystals growing in the sand-like "saccharoidal anhydrite" layer above saltdomes. The larger crystals are usually heavily included with unoriented anhydrite and dolomite "sand".

[John S. White]

As are the benitoites from San Benito Co. that are filled with crossite.

[Duncan Miller]

This back-lit smoky quartz with aegerine from Malosa, Malawi is reminiscent of Peter's quartz/tourmaline "sand" crystal, although not as congested. The aegerine crystals penetrate and poke out of all sides of the quartz crystal. The quartz is a "healed" floater, although obviously at one time attached to something, which grew in a granitic pegmatite (Cairncross, B. 2004. Aegerine and associated minerals from Mount Malosa, Malawi. South African Lapidary Magazine 36(2):7-15). The individual aegerine crystals are not contiguous, and could not have been a heap of crystals in which the quartz subsequently grew. Most of them are terminated, some of them doubly where they penetrate right through the quartz crystal. I suspect the mode of formation of this quartz-aegerine and Peter's quartz-tourmaline is very different from that of the gypsum "desert rose" type "sand" crystals.

Duncan