What you see here on this old-timer from the Mex-Tex, out of an old collection, is a very large cube (3.5 cm across) of galena with a coating of the lead oxide plattnerite on it. A smaller, similar cube is over towards the edge of the specimen. This is not common; it is a lot more typical to see a druse of cerussite (another lead mineral) than it is plattnerite, from this area. You can see traces of a copper mineral, likely brochantite or perhaps malachite, on and near the smaller cube (perhaps an alteration in progress). The quartz matrix makes an attractive backdrop to this significant New Mexico Galena crystal!